[LelouBil]

I think this is misguided.

> a PDF viewer can't even independently choose its own text and background color without the desktop environment messing with it

It can, it just chose to reuse the theming settings of it's platform (be it QT or GTK, or whatever) instead of forcing them.

The desktop environment is simply setting the defaults for the platform and most applications use them instead of forcing colors and themes and font.

And you can just try many combinations of themes + window managers + whatever app that fits in a desktop environment you want.

Maybe what's exactly to your liking doesn't exist yet, but the beautiful part it that it's a lot easier to make it for Linux than for any other OS because everything is open.