Change to my theme after syncing

[Odin2]

My Fx theme [SebaArt Blue Dream] has partially failed after I used Fx's sync following a Windows crash. (The horizontal line that is supposed to appear only under my inactive tabs now appears under my active tab as well. It corrects itself upon a browser restart but only briefly.) I thought that perhaps this aberration was connected with the humongous number of old bookmarks that unexpectedly inserted themselves after the sync, but I deleted those, and I'm told that any such bookmark deletion in Win10 is absolute: the bookmarks don't go to the recycle bin. I've found a similar theme failure with other themes. How can I make my theme behave properly?