It's the weekend so responses are delayed on SR.
I'm going to guess this is still the same bug that has plagued the system since 4.x.x was released. So, with in mind, I'll try to replicate the steps I did in order to get the CSS back.
If you can navigate pressing the highlighted links, go to Themes, and try to find the link that will enable 'DESIGNER MODE' let it run, and it should refresh the page. This should hopefully fix the issue, then turn off DESIGNERS MODE.