Ok, I think I partially fixed it. When I applied it, Curtains didn't change the browser on my right monitor but when I moved it to the center, it adopted the theme.
However, the Start button and the taskbar on both outside monitors are not the same as the theme on the center. I'll look through the settings in DisplayFusion and see what I can find.
EDIT: Yup that was it. Apologies for the premature posting. I should have played with it some before posting. We're good!