Answered, thanks Ketho for telling me about Immersion addon at curseforge. (Forum not letting me post the curseforge link even though it let me post another link below.)
So people can skip my wall of text now if they want. This thing converts the gossip windows into talking head windows that have white text on black background. (edit: mistakenly said it auto-accepts. It just accepts when you click on the dialog.) Just put it in the addon folder and runaway, no configuration needed.
Original post had some tips about gamma/brightness to see in difficult places too, down near the end, if anyone wants:
Hi, I have a glare sensitivity kind of issue where bright backgrounds washout dark text - about like trying to read the writing on a 200 watt light bulb that’s turned on. has anyone worked out how to flip the quest/gossip window’s background and font color scheme when talking to NPCs from dark font on gold background to light font on black background? I googled a lot and have encountered some RP gossip windows like tablets and shrines that have dark backgrounds and light text in their dialog windows when I interact with them. I’d love it if I could make all the human NPCs quest/gossip/dialog boxes have a dark background with light text.
I found this link on old EU forum post where someone provided code from scratch for an adhoc addon to adjust font sizes in the gossip frames independent of the rest of the UI.
The coder didn’t understand why black on gold would be hard to read and didn’t take a crack at that, but his other code would land people in the right place I think. At worst maybe a way to modify the font color to RGB code like 800 800 800 (or hex or whatever code the game uses for font colors). And for the background, since it’s a picture/image, maybe select the tablet gossip window background image instead of the npc gossip window background image or something like that.
Present workaround is alt-tab to wowhead. I’m not a hardcore RP’er super-bent on immersion, but-tabbing frequently still breaks the mood of the game a bit. Then the temptation to scroll down 2 more inches to see the final solutions in comments before trying to solve it myself…
Edit: also I’m pretty sure I’ve exhausted the in-game options like color blind mode and system-advanced-gamma/brightness/contrast stuff. Here are some cool settings I found in case anyone else feels a need-
Those dark caves - System->Advanced gamma to max, brightness to about 30% give or take. I actually use this most everywhere and the topography on the minimap is quite nice.
Seeing tiny quest objects in the foggy swamp, desert or snow at high noon - brightness max, gamma very low. Monsters and terrain are shadowy, but those dumb little emerald shards in the one place or the tiny pearls in the other are visible for miles around! Just blindly tab-target 12345 the trash while getting those quest object. This also helped immensely with the minimap while in stormwind and with reading world maps in classic wow stress test. (Stormwind quite different in wow classic.) Flipping gamma and brightness back and forth every single time I talk to someone though… alt-tab to wowhead is preferable to that. Hoping to just find a way to customize the gossip box fonts and background colors.
Those blue shards on the blue bottom of the blue bay wtf! 
- system->camera, turn off the water collision and fix the camera above the water looking down and the object outlines pop. Might need to flip gamma and brightness one way or the other too. (I got that one from wowhead comments about a quest in suramar.)
