Hi Jornes,
thanks a lot! I tried hard to reproduce the disappearing custom CSS issue (as you can see in my video), but unfortunately (or luckily 
), I couldn’t.
On Timo’s site, a JS error of a plugin causes the problem most likely - but on yours, there are no visible errors (which is good).
For now, I think we have no choice but to wait and see if the problem comes up again.
If it happens again, please make a screen video with the Developer Console open so that we can understand what is going on.