I honestly haven't had time to work on CSS file-handling.  Certainly, if you want to submit a PR to the upstream project suggesting a reasonable solution, I would try to review and merge it as quickly as possible.  Or maybe @jherland, who contributed the CSS/HTML stuff, as a few spare cycles?

Otherwise, just plop it down in a doc directory or whatever.  It is not a critical part of `git-imerge`, so IMHO the poor CSS file handling shouldn't be considered a showstopper bug.