How can I change the background color of documents in Ubuntu?

Are you asking about a specific type of document? The ability to change background color might depend on the app you're using, like Evince or LibreOffice. Each application has its own settings, so look in those to find the option you need.

It really depends on which application you're using. If the app has a night mode, it might turn the background to gray or black. But there isn't a universal system-wide setting for this, so you'll need to check each app individually.

If you're using Evince, which is the default document viewer on Ubuntu, there's a "Night Mode" option in the top-right hamburger menu. This feature inverts the colors, so you get black backgrounds with white text, which could help reduce eye strain.