Css

Can anyone tell me why the background-color value is being ignored on this page? (Please excuse the mess when viewing source.) It had been working fine many months (ie the background was blue; the text was white), and then — without me making any changes to the page — it started displaying as a white page. I've exhausted my own skills in trying to get the background set to #0165ff again, and I'm starting to wonder whether I've broken something further upstream: style.css; head.html or similar.

There seems to be an issue with modifying stylesheets. Every time I try to save changes to a css file (in any of my themes), I get this: 500 Internal Server Error Any help with this would be appreciated. Thanks!