I eventually figured it out, by way of a hacky workaround (which I guess for a ROM hack is kind of OK 😅).
In the interests of not being that guy who says "EDIT: Figured it out" and then leaves, here's how it works. Remember that this is (probably?) only relevant if you want to have a white background for your battle textbox.
1. Edit
graphics\battle_interface\textbox.png (in an image editor) and
graphics\battle_interface\textbox_map.bin (in Tilemap Studio) to whatever you want.
2. Edit
graphics\battle_interface\textbox_0.pal (in a text editor). Honestly, this is a bit of a pain in the butt and will require a lot of trial and error on your part. At the very least, the bottom value should be set to your chosen background colour (so,
255 255 255 for white), and the tenth value from the bottom should be set to your chosen text colour. It won't look right if you build at this point, but don't worry.
NOTE: textbox_0.pal controls the colours of the textbox tiles, the text printer itself,
and the "read more" arrow (
graphics/fonts/down_arrow_alt.png). So, yeah, trial and error.
EVEN MORE IMPORTANT NOTE: If you're trying to change the design of the "FIGHT/BAG/POKéMON/RUN" windows (etc.), it took me a long time to realise that even if you've completely changed their appearance in Step 1, they will stubbornly continue to use the colour palette from
graphics\text_window\1.png. That doesn't necessarily mean they have to be the same colour, just that they'll need to share a 16-colour palette. Opening up
1.png in Photoshop and experimenting with Image > Mode > Color Table should, eventually, get you there.
3. Now open up
src\battle_message.c (in a text editor) and replace all instances of
.fgColor = TEXT_COLOR_WHITE with
.fgColor = TEXT_COLOR_GREEN. That'll make the text use that tenth-from-the-bottom value we set earlier.
LET'S END WITH ONE MORE SPANNER IN THE WORKS: I'm getting rid of text shadows and going for a flatter, more modern look (and larger text), so I imagine this whole thing will be even more annoying if you have shadow colours to deal with as well. Sorry.
Very much still a WIP, don't judge me. 😅 If anyone, um, competent wants to write this guide up properly, you are more than welcome.