I tried switching skins in case it was a CSS problem and it's the same with each. CSS hacks can be used for defacement and it warranted a bug report. Glad you're here.
Edit:// You are correct it does work with default skin, only one I didn't switch to.
Edit2 If I didn't know any better I wouldn't be concerned the board is under attack, now missing same forum category and CSS assets(white background). I'd check the background CSS first. Admins should change passwords just to be on the safe side.
Edit3: seems fixed, before the arrow icon to expand category was not working.
And the background CSS is not fixed. Still white background. There were white tabs before, that's fixed.