Merging all issues about broken vSAN and VUM into this one.
This issue is blocking since all Add-ons (vSAN, VUM, vRealize, etc) are loaded as iFrame. I have not yet found an easy and reliable way to hook into iFrame creation. Hence I can't hook a custom CSS Sheet into the frame.
BeryJu
commented
6 years ago 
Merging all issues about broken vSAN and VUM into this one.
This issue is blocking since all Add-ons (vSAN, VUM, vRealize, etc) are loaded as iFrame. I have not yet found an easy and reliable way to hook into iFrame creation. Hence I can't hook a custom CSS Sheet into the frame.