Themes

I would fix the CSS first (remove the invalid comment characters and so edit the CSS to designate yellow for the navbar). If I remove the "//" characters using Firefox's DOM inspector, the navbar background changes to yellow.

However, the links in the navbar are still using the flatly.css values, so the CSS the specifies them should be added to yellow.css (I strongly suggest using your browser's dev tools' DOM inspector to identify the CSS that's styling those links and any other text that appears against the yellow background.)

As far as minification is concerned, there normally isn't a difference in the applied CSS between minified and not minified versions of the stylesheets in terms of effect, although invalid CSS or stray characters can cause a difference.

-- Gary