I had this proble for quite some time, but I always thought it was due to interactions between extensions and other stuff, but... while FG Light theme looks very pretty, white text in Hotbar makes it very hard to read.
Other parts of theme are okay (I didn't notice anything unusual), but even fresh campaign with nothing but theme loaded has this problem. I tried it with 5E and PF2 and it was the same (screenshots attached).

Screenshot_Clean_Light_5E.png
Screenshot_Clean_Light_PF2e.png

Thanks to text's shadow it is not that bad, but during a game, I sometimes spend precious moments trying to decipher Hotbar for shortcuts I prepared before.