In case you didn't know this: OpenWGA 7.3 suppports SCSS.
SCSS is a CSS preprocessor compiling your SCSS code to CSS.
To use SCSS define you scss code as CSS-recource (in scripts/css) with file extension .scss. Address this resource as usual using <tml:url type="css">. OpenWGA will find your scss code and compiles it to CSS on the fly without any further actions for the designer. No separate build process is required.
SCSS code is cached. As long as you don't change your scss code (or one of the "@import-ed" resources) the CSS is served from the cache without running the scss processor again.
In production mode the resulting CSS code is automatically compressed. In developer mode (using OpenWGA Developer Studio) the CSS is not compressed.