This is wonderful. My eyes have problems with white backgrounds and white on black is also problematic. But I was easily able to set it to a light grey background with black print. Now, if only it would work on non-browser pages (e.g. Excel, Word, all Windows 10 menu pages... that would be heaven!)
By the way, I ticked the box "Do not set "Override Document Colors" to "never" (not recommended)" which, with its double negatives means "set it to always".
March 15 2025: Firefox (on my PC) has disabled this app and I cannot download the latest version because I am being told "it appears to be corrupt". :-(