New Typeface
I hate to be a wet blanket when new things are implemented in Zotero, but here goes: the new typeface with 4.0.29.6 that is used with headers, the keyword panel on the left, and with the panel on the right side of the screen, makes my already out-of-focus eyesight all the more ineffective. Maybe if there was an option to switch these off, choose a different font or at least lose the engraved look, that would be helpful. Sorry ...
https://www.zotero.org/support/known_issues
On my Mac using the FF version of Zotero there _was_ a change to the typeface. I think this occurred with a recent FF update -- not with the recent Zotero update. At least, in my case this affects all FF-viewed websites and dialog boxes. The weight and darkness of the sans-serif characters seemed to change to less and lighter (not so much the size). I found it annoying and changed my accessibility settings for contrast among other things. The accessibility changes fixed the problem for me.
Themes don't affect Zotero Standalone, since it's a totally separate program from Firefox.