I am assuming that the issue that you're trying to test would be that the CSS changed in some incompatible way with the layout causing, for example, text to be truncated or otherwise visually "broken". If that's the case, then I would say that there isn't a good way to test the aesthetics of a page at this time. One of the primary benefits of TDD and CI is quick feedback so that you know something is broken before it gets to production. Not knowing much context around your environment and how those changes make it into your app it's hard to suggest solutions, but here is an example of a potential non-traditional option:
Put a commit hook into your repository that let's everyone on the team know via an e-mail when someone changes some CSS. Preferably with a diff of the CSS. This would give the team a heads up to keep an eye out for layout problems.
We started an experiment to use WATIR to walk some of the main screens in the app and take a picture using ImageMagik (essentially a screenshot) and store it in a "Last Known Good" folder. Every day re-run the script on a clean install of the app and data and place the images in a "Current" folder. At the end of each run use an MD5 to compare the images and alert on changes. Have the QA team review a list of flagged screenshots and if the change was acceptable (for example, a field was added as part of a feature) then copy "Current" to "Last Known Good". Unfortunately we didn't get our experiment finished so I don't know if it will work well. I'm concerned about the brittleness of screenshots as "assertions".
Hope that helps!
