
"System theme". Hmm... That is where I'm at.
Can't say I see what you've shown in your screenshots.
I'll note that on my end, Win7, I've got "fancy, schmancy" ("Aero", maybe it's called in) Windows crap turned off (in that my Win7 looks pretty much like XP).
I'm using (Windows) "Windows Classic" theme.
I believe ? you're using one of the (Windows) "High Contrast" themes ? & maybe that is affecting how/why FF is displaying as you're seeing?
If you change from (FF) "Sytem" to one of the other FF "themes", does the issue persist?
Or if you disable (Windows) "High Contrast" for "Windows Classic", does the issue persist?
(There used to be a time, when there were actual FF theme makers & actual FF themes, many of which were rather good.
To call this crud a "theme" is a true disservice to all the actual theme developers.)