At Rules are stripped from output stylesheet

Problem/Motivation

At Rules are stripped from output stylesheet.
StylesheetGenerator::generateCss strips all surrounding rules by calling CSSBlockList::getAllDeclarationBlocks() (including "AtRuleBlockList").

So this rule will work :

.d-md-none {
  @media (min-width: 768px) {
    display: none !important;
  }
}

But not this one :

@media (min-width: 768px) {
  .d-md-none {
    display: none !important;
  }
}

Steps to reproduce

• install ui_suite_bootstrap
• install ui_styles
• Add this div anywhere <div class="d-block d-md-none">TEST</div>
• The div should hide in desktop but does not

Proposed resolution

Do not get blocks with getAllDeclarationBlocks() method. Process level 1 content of parsed css separately depending on their type. For AtRuleBlockList items, process included blocks and generate css by calling render on the AtRule not on each child.