diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 3483330..41ba593 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -2684,18 +2684,18 @@ function template_preprocess_html(&$variables) {
 
   // Add information about the number of sidebars.
   if (!empty($variables['page']['sidebar_first']) && !empty($variables['page']['sidebar_second'])) {
-    $variables['attributes']['class'][] = 'two-sidebars';
+    $variables['attributes']['class'][] = 'l-two-sidebars';
   }
   elseif (!empty($variables['page']['sidebar_first'])) {
-    $variables['attributes']['class'][] = 'one-sidebar';
-    $variables['attributes']['class'][] = 'sidebar-first';
+    $variables['attributes']['class'][] = 'l-one-sidebar';
+    $variables['attributes']['class'][] = 'l-sidebar-first';
   }
   elseif (!empty($variables['page']['sidebar_second'])) {
-    $variables['attributes']['class'][] = 'one-sidebar';
-    $variables['attributes']['class'][] = 'sidebar-second';
+    $variables['attributes']['class'][] = 'l-one-sidebar';
+    $variables['attributes']['class'][] = 'l-sidebar-second';
   }
   else {
-    $variables['attributes']['class'][] = 'no-sidebars';
+    $variables['attributes']['class'][] = 'l-no-sidebars';
   }
 
   // Populate the body classes.
@@ -3059,14 +3059,14 @@ function template_preprocess_maintenance_page(&$variables) {
     $variables['attributes']['class'][] = 'db-offline';
   }
   if ($variables['layout'] == 'both') {
-    $variables['attributes']['class'][] = 'two-sidebars';
+    $variables['attributes']['class'][] = 'l-two-sidebars';
   }
   elseif ($variables['layout'] == 'none') {
-    $variables['attributes']['class'][] = 'no-sidebars';
+    $variables['attributes']['class'][] = 'l-no-sidebars';
   }
   else {
-    $variables['attributes']['class'][] = 'one-sidebar';
-    $variables['attributes']['class'][] = 'sidebar-' . $variables['layout'];
+    $variables['attributes']['class'][] = 'l-one-sidebar';
+    $variables['attributes']['class'][] = 'l-sidebar-' . $variables['layout'];
   }
 
   // Dead databases will show error messages so supplying this template will
diff --git a/core/themes/bartik/css/layout.css b/core/themes/bartik/css/layout.css
index 8c1ff95..4bcfa0e 100644
--- a/core/themes/bartik/css/layout.css
+++ b/core/themes/bartik/css/layout.css
@@ -153,22 +153,22 @@ body,
     float: left; /* LTR */
     position: relative;
   }
-  .two-sidebars #content {
+  .l-two-sidebars #content {
     margin-left: 25%;
     margin-right: 25%;
     width: 50%;
   }
-  .one-sidebar #content {
+  .l-one-sidebar #content {
     width: 75%;
   }
-  .no-sidebars #content {
+  .l-no-sidebars #content {
     width: 100%;
   }
-  .sidebar-first #content {
+  .l-sidebar-first #content {
     margin-left: 25%;
     margin-right: 0;
   }
-  .sidebar-second #content {
+  .l-sidebar-second #content {
     margin-right: 25%;
     margin-left: 0;
   }
diff --git a/core/themes/bartik/css/print.css b/core/themes/bartik/css/print.css
index fbe386a..8053581 100644
--- a/core/themes/bartik/css/print.css
+++ b/core/themes/bartik/css/print.css
@@ -8,10 +8,10 @@ select {
   color: #000;
   background: none;
 }
-body.two-sidebars,
-body.sidebar-first,
-body.sidebar-second,
-body {
+body,
+.l-two-sidebars,
+.l-sidebar-first,
+.l-sidebar-second {
   width: 640px;
 }
 #sidebar-first,
@@ -23,8 +23,8 @@ body {
 .add-or-remove-shortcuts {
   display: none;
 }
-.one-sidebar #content,
-.two-sidebars #content {
+.l-one-sidebar #content,
+.l-two-sidebars #content {
   width: 100%;
 }
 #triptych-wrapper {
diff --git a/core/themes/stark/css/layout-rtl.css b/core/themes/stark/css/layout-rtl.css
index d5e9c6d..78d0228 100644
--- a/core/themes/stark/css/layout-rtl.css
+++ b/core/themes/stark/css/layout-rtl.css
@@ -1,24 +1,22 @@
 @media all and (min-width: 480px) and (max-width: 959px) {
-  body.sidebar-first #content,
-  body.two-sidebars #content {
+  .l-sidebar-first #content,
+  .l-two-sidebars #content {
     float: left;
   }
-  body.sidebar-second #content {
+  .l-sidebar-second #content {
     float: right;
   }
-
   #sidebar-first {
     float: right;
   }
-
   #sidebar-second {
     float: left;
   }
-  body.two-sidebars #sidebar-second {
+  .l-two-sidebars #sidebar-second {
     float: right;
     clear: right;
   }
-  body.two-sidebars #sidebar-second .block {
+  .l-two-sidebars #sidebar-second .block {
     float: right;
   }
 }
@@ -27,25 +25,23 @@
   #content {
     float: right;
   }
-  body.sidebar-first #content {
+  .l-sidebar-first #content {
     left: 0;
     right: 20%;
   }
-  body.two-sidebars #content {
+  .l-two-sidebars #content {
     left: 0;
     right: 20%;
   }
-
   #sidebar-first {
     float: right;
     left: 0;
     right: -80%;
   }
-  body.two-sidebars #sidebar-first {
+  .l-two-sidebars #sidebar-first {
     left: 0;
     right: -60%;
   }
-
   #sidebar-second {
     float: left;
   }
diff --git a/core/themes/stark/css/layout.css b/core/themes/stark/css/layout.css
index 39f533b..d6b297c 100644
--- a/core/themes/stark/css/layout.css
+++ b/core/themes/stark/css/layout.css
@@ -28,35 +28,34 @@ img {
   #content {
     width: 100%;
   }
-  body.sidebar-first #content,
-  body.two-sidebars #content {
+  .l-sidebar-first #content,
+  .l-two-sidebars #content {
     width: 67%;
     float: right; /* LTR */
   }
-  body.sidebar-second #content {
+  .l-sidebar-second #content {
     width: 67%;
     float: left; /* LTR */
   }
-
   #sidebar-first {
     width: 33%;
     float: left; /* LTR */
   }
-
   #sidebar-second {
     width: 33%;
     float: right; /* LTR */
   }
-  body.two-sidebars #sidebar-second {
+  .l-two-sidebars #sidebar-second {
     clear: both;
     width: 100%;
   }
 
-  body.two-sidebars #sidebar-second .block {
+  /* Layout the second sidebar's blocks in 3 columns. */
+  .l-two-sidebars #sidebar-second .block {
     float: left; /* LTR */
     width: 33%;
   }
-  body.two-sidebars #sidebar-second .block:nth-child(3n+1) {
+  .l-two-sidebars #sidebar-second .block:nth-child(3n+1) {
     clear: both;
   }
 }
@@ -66,27 +65,25 @@ img {
     width: 100%;
     float: left; /* LTR */
   }
-  body.sidebar-first #content {
+  .l-sidebar-first #content {
     width: 80%;
     left: 20%; /* LTR */
   }
-  body.sidebar-second #content {
+  .l-sidebar-second #content {
     width: 80%;
   }
-  body.two-sidebars #content {
+  .l-two-sidebars #content {
     width: 60%;
     left: 20%; /* LTR */
   }
-
   #sidebar-first {
     width: 20%;
     float: left; /* LTR */
     left: -80%; /* LTR */
   }
-  body.two-sidebars #sidebar-first {
+  .l-two-sidebars #sidebar-first {
     left: -60%; /* LTR */
   }
-
   #sidebar-second {
     float: right; /* LTR */
     width: 20%;