Short of custom themes perhaps, I know of no way to directly change the selection color.
Some options to improve visibility, in increasing order of difficulty/inconvenience:
1. Try switching to Dark theme, it's a bit more obvious. (This is in Settings:General)
2. The Thumbnails (only) or Icons views can be subtle in this regard. Change your view mode in the browser (View->View As) from Thumbnails (only) or Icons (only) to Thumbnails+Filename/Labels/Details or literally any other mode. When one of the other modes is in use, the selection will also highlight the text that accompanies the icon/thumbnail, which is very conspicuous. If you can't tell the difference then, you're blind.
3. Make sure the Background and Border color of the Thumbnail cell are set to something distinct from the blue selection color. (Settings:Thumbnail:Appearance). If you have those set to blue it will make selections near-impossible to see, but that's user error.
4. Experiment with a custom theme. You can define colors directly. I don't use it, so I can't give you any additional detail. I have seen a comment from Pierre that that color "comes from the system" and so can't be changed, but I don't know if that applies to custom themes. I have never seen any actual documentation, and discussions on the forums of the specifics of modifying themes seem to be almost nonexistent. Here are a couple of very meager threads:
https://newsgroup.xnview.com/viewtopic.php?t=33042
https://newsgroup.xnview.com/viewtopic. ... es#p204339
Whether you can be successful with a custom theme, I don't know. There's a paucity of custom themes to download, so examples are lacking.
You might possibly get somewhere reading general documentation about Qt stylesheets (Qt is the development framework for XnViewMP):
https://doc.qt.io/archives/qt-5.15/stylesheet.html
Some people may have a strong preference regard light vs dark theme. That's fine. So changing your view (#2) might be most appropriate. I use Thumbnails+Labels, and it is not possible for a sighted person to not clearly see the selection in that mode, especially with the Dark theme.
If you feel that your vision is normal and #1 or #2 don't work, you'd better post a screenshot illustrating the situation so we can see what's wrong.
Hopefully this will help...