When I select "use selection color (plain)" menu highlight (in oxygen-settings) black fonts change to white on highlight for KDE apps, but they stay black in GTK apps, making them difficult to read. To avoid this I could simply use other highlight options but I quite like plain selection highlight. Any way to fix this?

One possibility is that this feature is simply not implemented in the Oxygen Gtk style, or that Gtk does not permit styles to implement this sort of functionality.

Does changing the highlight colour to something else have any impact, or does it have effect if used under a different colour scheme?

Does "gtk+ apps" mean "gtk+ apps" or "firefox/libreoffice"?

Afaiu, gtk+ does not use/permit styles to paint the text, so it depends on whether "the widget does it right" and out of a long experience i can assure you that developers of esp. custom widgets (what easily happens on gtk+) and esp. those operating on "metastyles" (ie. bringing their own style engine, operating on the system ones - LO next to Opera would be candidates here for Qt) really *love* to hardcode colors

If the element does not change to the proper FG color with any other gtk+ style, you're likely screwed.

I see. I hadn't checked other apps besides Firefox and Libreoffice, but now that I do, not all exhibit this hardcoded menu-color-on-highlight. I thought hardcoding colors (something websites do a lot) is a fashion faux pas in the design world. I'm surprised to see big names like Mozilla and The Document Foundation doing it.