When developing surveys and their themes, I always try to keep the code clean. Would it be an idea to implement the possibility to always use the same theme, but per survey ask for an extra CSS that implements extra's for that theme?

Example: you are working on survey xxxxx with theme yyyyy. When showing this survey it uses all CSS-files from theme yyyyy plus an extra stylesheet called xxxxx.css. Everything that is special for survey xxxxx goes in xxxxx.css

Another approach could be:
The body tag for survey xxxxx has an extra class: .surveyxxxxx This extra class can be used to make things special for surveyxxxxx
Same can be done for groups: body tag for groupdddddd gets another class: .groupddddd

Any thoughts?

Yep,

I think one time we think of a /upload/surveys/XXXX/template.css and /upload/surveys/XXXXX/template.js

But, now, just the time to do it

In general this is a nice idea. You can add this as a feature request at our feature tracker . Please describe your idea as detailed as possible and try to outline a suitable solution/implementation as well.