Problem/Motivation
We currently have a slightly-awkward CSS aggregation strategy. Our theme-level CSS is artificially forced to be loaded after all module-level CSS. That means we have twice as many aggregated CSS files as are necessary.
Proposed resolution
After we refactor our module CSS to be reusable components with low selector specificity, we can remove CSS_AGGREGATE_THEME aggregate file and replace it with a 'weight' inside the existing CSS_AGGREGATE_DEFAULT file.
This is the seventh step in a proposed solution outlined at #1921610: [Meta] Architect our CSS.
Remaining tasks
This patch is blocked by step 6 (and its sub-parts) of #1921610: [Meta] Architect our CSS.
After this issue's completion, we still have all the other steps outlined at #1921610: [Meta] Architect our CSS.
User interface changes
none
API changes
Removes the CSS_AGGREGATE_THEME constant.