I have sensitive eyes and the stark white background of the forums make them unbearable for me to read.
Can't, if you want write an HTML plugin to automatically change all "white" XenoForo backgrounds to "Black" and all Text to "White"
Actually they can, users have the ability to change the theme on their side based on installed themes. You may be able to use the default xenforo theme as that's not white but rather a blue-ish colour so shouldn't be as bad on your eyes
There are quite a lot of adjustments we have made to the theme and templates. Adding a dark theme won't work out of the box. It would require the same adjustments and would need to be maintained. Therefor this is a no. But you can look at Tampermonkey • Home and a like.
depending on the browser you use, you might try a plugin for example for google chrome => High Contrast - Chrome Web Store