ok... next thing is, I created a new topic in the SS folder thinking if it acted like it did with the other topic, it would just drop the css into the same folder (this time Stylesheet where the new topic is)... but then it came up and asked me if i wanted to overwrite the existing Sol8. css.. so i said yes... and it still didn't show up in the SS folder.
If i unlink the css "already being used in the project" from the topics its linked to, it disappears again...
if i go into a Topic's properties and try to use the browse link to browse to the Resource/SS folder of the same project, it tells me it's already there and i can't add it again... but i still can't see it in PM.
I have tried every combo i can think of.. and deleted the CPD every other time... good lord!
Actually, you might find that deleting the cpd is causing the css file to vanish. (I've seen it affect some (but not all) images as well, and in fact was a major reason my last job switched to Madcap Flare.)
Try this:
(NOTE: if it's a really large project, you might need to go out for coffee or lunch. Basically this will refresh the RH file list database (my interpretation), which can take a long time for large projects. We had one that took an hour.)
- Create a new folder. It can be anywhere and called anything and you can delete it in a moment.
- Check if your css file is displaying in Project Manager again.
- Delete the new folder you just created.
If this works, you'll have to create and delete a new folder when you want to edit the stylesheet after you delete the cpd (or just use windows explorer and an external editor).
Please also report this as a bug to Adobe. I've done so on a couple of occasions, but most people look at me like I'm mad.
Which reminds me, I should report it again and link to this thread. 
If it doesn't work, sorry, I'm out of ideas.
(Edit: Bug submitted with link to this thread and also https://forums.adobe.com/message/3478909)