diff --git a/core/modules/system/css/system.admin.css b/core/modules/system/css/system.admin.css
index c600539..b627bb0 100644
--- a/core/modules/system/css/system.admin.css
+++ b/core/modules/system/css/system.admin.css
@@ -6,51 +6,51 @@
 /**
  * Administration blocks.
  */
-div.admin-panel {
+.admin-panel {
   margin: 0;
   padding: 5px 5px 15px 5px;
 }
-div.admin-panel .description {
+.admin-panel .description {
   margin: 0 0 3px;
   padding: 2px 0 3px 0;
 }
-div.admin-panel .body {
+.admin-panel .body {
   padding: 0 4px 2px 8px; /* LTR */
 }
-[dir=rtl] div.admin-panel .body {
+[dir=rtl] .admin-panel .body {
   padding: 0 8px 2px 4px;
 }
-div.admin {
+.admin {
   padding-top: 15px;
 }
 @media screen and (min-width: 40em) {
-  div.admin .left {
+  .admin .left {
     float: left; /* LTR */
     width: 47%;
     margin-left: 1em; /* LTR */
   }
-  [dir=rtl] div.admin .left {
+  [dir=rtl] .admin .left {
     float: right;
     margin-left: 0;
     margin-right: 1em;
   }
-  div.admin .right {
+  .admin .right {
     float: right; /* LTR */
     width: 47%;
     margin-right: 1em; /* LTR */
   }
-  [dir=rtl] div.admin .right {
+  [dir=rtl] .admin .right {
     float: left;
     margin-left: 1em;
     margin-right: 0;
   }
 }
-div.admin .expert-link {
+.admin .expert-link {
   text-align: right; /* LTR */
   margin-right: 1em; /* LTR */
   padding-right: 4px; /* LTR */
 }
-[dir=rtl] div.admin .expert-link {
+[dir=rtl] .admin .expert-link {
   margin-right: 0;
   margin-left: 1em;
   padding-right: 0;
@@ -78,37 +78,37 @@ small .admin-link:after {
 /**
  * Modules page.
  */
-#system-modules table {
+.system-modules table {
   table-layout: fixed;
 }
-#system-modules div.incompatible {
+.system-modules div.incompatible {
   font-weight: bold;
 }
-#system-modules th.checkbox {
+.system-modules th.checkbox {
   width: 4%;
 }
-#system-modules th.name {
+.system-modules th.name {
   width: 25%;
 }
-#system-modules td {
+.system-modules td {
   vertical-align: top;
 }
-#system-modules .expand .inner {
+.system-modules .expand .inner {
   background: transparent url(../../../misc/menu-collapsed.png) left .6em no-repeat;
   margin-left: -12px;
   padding-left: 12px;
 }
-#system-modules .expanded .expand .inner {
+.system-modules .expanded .expand .inner {
   background: transparent url(../../../misc/menu-expanded.png) left .6em no-repeat;
 }
-#system-modules label {
+.system-modules label {
   color: #1d1d1d;
   font-size: 1.15em;
 }
-#system-modules .description {
+.system-modules .description {
   cursor: pointer;
 }
-#system-modules .description .inner {
+.system-modules .description .inner {
   color: #5c5c5b;
   height: 20px;
   line-height: 20px;
@@ -116,66 +116,66 @@ small .admin-link:after {
   text-overflow: ellipsis;
   white-space: nowrap;
 }
-#system-modules .expanded .description .inner {
+.system-modules .expanded .description .inner {
   height: auto;
   overflow: visible;
   white-space: normal;
 }
-#system-modules .expanded .description .text {
+.system-modules .expanded .description .text {
   -webkit-hyphens: auto;
   -moz-hyphens: auto;
   hyphens: auto;
 }

 @media screen and (max-width: 40em) {
-  #system-modules th.name {
+  .system-modules th.name {
     width: 20%;
   }
-  #system-modules th.description {
+  .system-modules th.description {
     width: 40%;
   }
 }
-#system-modules .requirements {
+.system-modules .requirements {
   padding: 5px 0;
   max-width: 490px;
 }
-#system-modules .links {
+.system-modules .links {
   overflow: hidden; /* prevents collapse */
 }
-#system-modules .checkbox {
+.system-modules .checkbox {
   margin: 0 5px;
 }
-#system-modules .checkbox .form-item {
+.system-modules .checkbox .form-item {
   margin-bottom: 0;
 }
-div.admin-requirements,
-div.admin-required {
+.admin-requirements,
+.admin-required {
   font-size: 0.9em;
   color: #666;
 }
-span.admin-disabled {
+.admin-disabled {
   color: #800;
 }
-span.admin-enabled {
+.admin-enabled {
   color: #080;
 }
-span.admin-missing {
+.admin-missing {
   color: #f00;
 }
-a.module-link {
+.links .module-link {
   display: block;
   padding: 2px 20px;
   white-space: nowrap;
   margin-top: 2px;
   float: left; /* LTR */
 }
-a.module-link-help {
+.links .module-link-help {
   background: url(../../../misc/help.png) 0 50% no-repeat; /* LTR */
 }
-a.module-link-permissions {
+.links .module-link-permissions {
   background: url(../../../misc/permissions.png) 0 50% no-repeat; /* LTR */
 }
-a.module-link-configure {
+.links .module-link-configure {
   background: url(../../../misc/configure.png) 0 50% no-repeat; /* LTR */
 }
 .module-help {
@@ -186,33 +186,33 @@ a.module-link-configure {
 /**
  * Status report.
  */
-table.system-status-report td {
+.system-status-report td {
   padding: 6px;
   vertical-align: top;
 }
-table.system-status-report td:nth-child(-n+2) {
+.system-status-report td:nth-child(-n+2) {
   background-color: rgba(0, 0, 0, 0.04);
 }
-table.system-status-report td.status-icon {
+.system-status-report .status-icon {
   width: 16px;
   padding-right: 0; /* LTR */
 }
-[dir=rtl] table.system-status-report td.status-icon {
+[dir=rtl] .system-status-report .status-icon {
   padding-left: 0;
   padding-right: 6px;
 }
-table.system-status-report td.status-icon div {
+.system-status-report .status-icon div {
   background-repeat: no-repeat;
   height: 16px;
   width: 16px;
 }
-table.system-status-report tr.error td.status-icon div {
+.system-status-report .error .status-icon div {
   background-image: url(../../../misc/message-16-error.png);
 }
-table.system-status-report tr.warning td.status-icon div {
+.system-status-report .warning .status-icon div {
   background-image: url(../../../misc/message-16-warning.png);
 }
-table.system-status-report .status-title {
+.system-status-report .status-title {
   width: 25%;
 }

@@ -395,21 +395,21 @@ table.system-status-report .status-title {
   border-left: 1px solid #cdcdcd;
   border-right: none;
 }
-.theme-selector .operations li.last {
+.theme-selector .operations .last {
   padding: 0 0 0 0.7em; /* LTR */
   border-right: none; /* LTR */
 }
-[dir=rtl] .theme-selector .operations li.last {
+[dir=rtl] .theme-selector .operations .last {
   padding: 0 0.7em 0 0;
   border-left: none;
 }
-.theme-selector .operations li.first {
+.theme-selector .operations .first {
   padding: 0 0.7em 0 0; /* LTR */
 }
-[dir=rtl] .theme-selector .operations li.first {
+[dir=rtl] .theme-selector .operations .first {
   padding: 0 0 0 0.7em;
 }
-#system-themes-admin-form {
+.system-themes-admin-form {
   clear: left;
 }

diff --git a/core/modules/system/css/system.diff.css b/core/modules/system/css/system.diff.css
index 1c73598..aee2d73 100644
--- a/core/modules/system/css/system.diff.css
+++ b/core/modules/system/css/system.diff.css
@@ -16,30 +16,31 @@
 /**
  * Inline diff markup
  */
-span.diff-deleted { color:#ccc; }
-span.diff-deleted img { border: solid 2px #ccc; }
-span.diff-changed { background:#ffb; }
-span.diff-changed img { border:solid 2px #ffb; }
-span.diff-added { background:#cfc; }
-span.diff-added img { border: solid 2px #cfc; }
+.diff-deleted { color:#ccc; }
+.diff-deleted img { border: solid 2px #ccc; }
+.diff-changed { background:#ffb; }
+.diff-changed img { border:solid 2px #ffb; }
+.diff-added { background:#cfc; }
+.diff-added img { border: solid 2px #cfc; }

 /**
  * Traditional split diff theming
  */
-table.diff {
+.diff {
   border-spacing: 4px;
   margin-bottom: 20px;
   table-layout: fixed;
   width: 100%;
 }
-table.diff tr.even, table.diff tr.odd {
+.diff .even,
+.diff .odd {
   background-color: inherit;
   border: none;
 }
-td.diff-prevlink {
+.diff-prevlink {
   text-align: left;
 }
-td.diff-nextlink {
+.diff-nextlink {
   text-align: right;
 }
 td.diff-section-title, div.diff-section-title {
@@ -48,36 +49,36 @@ td.diff-section-title, div.diff-section-title {
   font-weight: bold;
   padding: 0.1em 1em;
 }
-td.diff-context {
+.diff-context {
     background-color: #fafafa;
 }
-td.diff-deletedline {
+.diff-deletedline {
   background-color: #ffa;
   width: 50%;
 }
-td.diff-addedline {
+.diff-addedline {
   background-color: #afa;
   width: 50%;
 }
-span.diffchange {
+.diffchange {
   color: #f00;
   font-weight: bold;
 }

-table.diff col.diff-marker {
+.diff .diff-marker {
   width: 1.4em;
 }
-table.diff col.diff-content {
+.diff .diff-content {
   width: 50%;
 }
-table.diff th {
+.diff th {
   padding-right: inherit;
 }
-table.diff td div {
+.diff div {
   overflow: auto;
   padding: 0.1ex 0.5em;
   word-wrap: break-word;
 }
-table.diff td {
+.diff td {
   padding: 0.1ex 0.4em;
 }
diff --git a/core/modules/system/css/system.maintenance.css b/core/modules/system/css/system.maintenance.css
index 9fd9f38..0505028 100644
--- a/core/modules/system/css/system.maintenance.css
+++ b/core/modules/system/css/system.maintenance.css
@@ -25,21 +25,21 @@
 /**
  * Authorize.php styles
  */
-#edit-submit-connection {
+.edit-submit-connection {
   clear: both;
 }
-#edit-submit-process,
+.edit-submit-process,
 .filetransfer {
   display: none;
   clear: both;
 }
-.js #edit-submit-connection {
+.js .edit-submit-connection {
   display: none;
 }
-.js #edit-submit-process {
+.js .edit-submit-process {
   display: block;
 }

-#edit-connection-settings-change-connection-type {
+.edit-connection-settings-change-connection-type {
   margin: 2.6em 0.5em 0 1em;
 }
diff --git a/core/modules/system/css/system.module.css b/core/modules/system/css/system.module.css
index cf08f02..e0fb136 100644
--- a/core/modules/system/css/system.module.css
+++ b/core/modules/system/css/system.module.css
@@ -9,19 +9,19 @@
  * @see autocomplete.js
  */
 /* Suggestion list */
-#autocomplete {
+.autocomplete {
   border: 1px solid;
   overflow: hidden;
   position: absolute;
   z-index: 100;
 }
-#autocomplete ul {
+.autocomplete ul {
   list-style: none;
   list-style-image: none;
   margin: 0;
   padding: 0;
 }
-#autocomplete li {
+.autocomplete li {
   background: #fff;
   color: #000;
   cursor: default;
@@ -29,18 +29,18 @@
 }

 /* Animated throbber */
-.js input.form-autocomplete {
+.js .form-autocomplete {
   background-image: url(../../../misc/throbber.gif);
   background-position: 100% 2px; /* LTR */
   background-repeat: no-repeat;
 }
-.js[dir=rtl] input.form-autocomplete {
+.js[dir=rtl] .form-autocomplete {
   background-position: 0% 2px;
 }
-.js input.throbbing {
+.js .throbbing {
   background-position: 100% -18px; /* LTR */
 }
-.js[dir=rtl] input.throbbing {
+.js[dir=rtl] .throbbing {
   background-position: 0% -18px;
 }

@@ -86,10 +86,10 @@
  *
  * @see tabledrag.js
  */
-body.drag {
+.drag {
   cursor: move;
 }
-.draggable a.tabledrag-handle {
+.draggable .tabledrag-handle {
   cursor: move;
   float: left; /* LTR */
   height: 1.7em;
@@ -97,22 +97,22 @@ body.drag {
   overflow: hidden;
   text-decoration: none;
 }
-[dir=rtl] .draggable a.tabledrag-handle {
+[dir=rtl] .draggable .tabledrag-handle {
   float: right;
   margin-right: -1em;
   margin-left: 0;
 }
-a.tabledrag-handle:hover {
+.tabledrag-handle:hover {
   text-decoration: none;
 }
-a.tabledrag-handle .handle {
+.tabledrag-handle .handle {
   background: url(../../../misc/draggable.png) no-repeat 6px 9px;
   height: 13px;
   margin: -0.4em 0.5em; /* LTR */
   padding: 0.42em 0.5em; /* LTR */
   width: 13px;
 }
-[dir=rtl] a.tabledrag-handle .handle {
+[dir=rtl] .tabledrag-handle .handle {
   margin: -0.4em 0.5em;
   padding: 0.42em 0.5em;
 }
@@ -123,44 +123,44 @@ a.tabledrag-handle .handle {
   display: inline-block;
   padding: 10px 0;
 }
-.touch a.tabledrag-handle {
+.touch .tabledrag-handle {
   height: 44px;
   width: 40px;
 }
-.touch a.tabledrag-handle .handle {
+.touch .tabledrag-handle .handle {
   background-position: 40% 19px;
   height: 21px;
 }
-.touch .draggable.drag a.tabledrag-handle .handle {
+.touch .draggable.drag .tabledrag-handle .handle {
   background-position: 50% -32px;
 }
-.no-touch a.tabledrag-handle:hover .handle,
-.no-touch a.tabledrag-handle:focus .handle {
+.no-touch .tabledrag-handle:hover .handle,
+.no-touch .tabledrag-handle:focus .handle {
   background-position: 6px -11px;
 }
-div.indentation {
+.indentation {
   float: left; /* LTR */
   height: 1.7em;
   margin: -0.4em 0.2em -0.4em -0.4em; /* LTR */
   padding: 0.42em 0 0.42em 0.6em; /* LTR */
   width: 20px;
 }
-[dir=rtl] div.indentation {
+[dir=rtl] .indentation {
   float: right;
   margin: -0.4em -0.4em -0.4em 0.2em;
   padding: 0.42em 0.6em 0.42em 0;
 }
-div.tree-child {
+.tree-child {
   background: url(../../../misc/tree.png) no-repeat 11px center; /* LTR */
 }
-div.tree-child-last {
+.tree-child-last {
   background: url(../../../misc/tree-bottom.png) no-repeat 11px center; /* LTR */
 }
-[dir=rtl] div.tree-child,
-[dir=rtl] div.tree-child-last {
+[dir=rtl] .tree-child,
+[dir=rtl] .tree-child-last {
   background-position: -65px center;
 }
-div.tree-child-horizontal {
+.tree-child-horizontal {
   background: url(../../../misc/tree.png) no-repeat -11px center;
 }
 .tabledrag-toggle-weight-wrapper {
@@ -175,7 +175,7 @@ div.tree-child-horizontal {
  *
  * @see tableheader.js
  */
-table.sticky-header {
+.sticky-header {
   background-color: #fff;
   margin-top: 0;
   z-index: 500;
diff --git a/core/modules/system/css/system.plugin.ui.css b/core/modules/system/css/system.plugin.ui.css
index c932788..af9b8d3 100644
--- a/core/modules/system/css/system.plugin.ui.css
+++ b/core/modules/system/css/system.plugin.ui.css
@@ -1,17 +1,17 @@
-#block-library .left-col,
-#block-library .right-col {
+.block-library .left-col,
+.block-library .right-col {
   float:left;
   width:66%;
   height:100%;
   background-color:#ffffff;
 }

-#block-library .right-col {
+.block-library .right-col {
   width:34%;
   background-color:#f7f7f7;
 }

-#block-library .right-col h3 {
+.block-library .right-col h3 {
   margin: 1em -20px;
   background-color:#d7d7d7;
   color:#333333;
@@ -19,11 +19,11 @@
   font-size:1.1em;
 }

-#block-library .inside {
+.block-library .inside {
   margin:0 20px;
 }

-#block-library .bottom-bar {
+.block-library .bottom-bar {
   width:100%;
   clear:both;
 }
diff --git a/core/modules/system/templates/system-plugin-ui-form.html.twig b/core/modules/system/templates/system-plugin-ui-form.html.twig
index 5987888..e92a291 100644
--- a/core/modules/system/templates/system-plugin-ui-form.html.twig
+++ b/core/modules/system/templates/system-plugin-ui-form.html.twig
@@ -13,7 +13,7 @@
  * @ingroup themeable
  */
 #}
-<div id="block-library" class="container">
+<div id="block-library" class="container block-library">
   <div class="left-col">
     <div class="inside">
       {{ form.left }}