My CSS only changed colors, not touched Menu structure. Besides 22h ago when I checked your site, menu was working fine exept colors. So I think it has something extra....
You can always rename custom.css file for 3 minutes and recheck.
It will be OK after refreshing the site, it means that you made "small" mistake in Custom CSS code, like missing "}" etc. Use Online CSS validator to be sure, or check line by line.
I know that we all make mistakes from time to time, but nobody said that webmaster life is easy.