Fix `getStyleSheet` CSS File Retrieval

Fix `View::getStyleSheet()` css file retreival

`.css` files are not retrieved from getRecord so this always returns
blank

Change `Environment::getOverrides()` to cache all files not just files
that end in .php
Merge pull request #985 from mkly/fix_overrides_cache_css_js_getstylesheet

Fix `getStyleSheet` CSS File Retrieval