For example I downloaded that skin.
https://vborg.vbsupport.ru/showthread.php?t=248257
Which is from what I can read for both, CMS & Forum. So I set the Forum to that theme, and in that "HOME" tab I set the Theme to be "inherit". But still, didn't had the new Theme for the CMS. Because my new Site is still in progress and basically a re-import of old Users & Posts of my old one, I checked my profile in the AdminCP and it was set to "Default Skin" which I think is ok, since the theme above is set to "Default" for my board now.
Only when I changed the Theme for my Profile in AdminCP, to the new one, then I can see it in the CMS. But that would mean i would have to set either every other user to that Theme as default or I dont know how else.
Its all very confusing. In every other Portal or CMS System the Theme you set as default, also shows as default for everyone. Here it seems all weird different.