OH NO!!!
It just happened again. CSS blanked.... I Guess I can go back to my workaround again!
Working with Sigil 8.4... yadda yadda (Windows 8.1, Firefox, windows explorer, and skype)
I was almost done with the book...already had styles.css imported and edited...I "Generated the table of contents" then realized I had not imported the sgc-toc.css so I did that. Then "created HTML toc".
The toc appeared formatted correctly per my imported sgc-toc file. I went in and manually edited some of the titles in the toc.html, then manually edited some of the titles in the toc.ncx (to match the toc.html).
In the preview pane the formatting for the toc appears as I had formatted it...but it was just a stale view. The toc file had closed itself from the code view and the sgc-toc.css file was still in the book browser section...but completely blank.
I re-opened the toc.html and all my manual edits were still there...and the file was still properly linked to the sgc-toc.css....just no formatting since the css file was blank.
I had not saved the book from the point right before importing the sgc-toc.css file.
Here's my sgc-toc.css template if anyone thinks that will help:
-- Hope that helps!