Does this only affect the Family view of the tree, or all pages on Ancestry? The message boards are apparently working for you, so I am assuming that it is not all pages...
I assume you have tried CTRL+F5, which forces a refresh from the server? That should force the browser to request the new CSS files from Ancestry.
The browser should update files from Ancestry automatically, but there are browser settings that will force the files to be updated.
1.Go to Tools
2.Select Internet Options.
3.Under the General tab, there should be an section for Browsing History or Temporary Internet files.
4.Click on Settings button in this section.
5.There should be a section for "Check for newer versions of stored pages"
6.Select "Every visit to the page."
7.Click OK/Apply to close the dialog(s)