@Alda Using Firefox's builtin web-dev tools, I don't see any background color set on that element, so it defaults to my desktop theme's current dark purple color for input boxes.

I see this problem more often, where CSS sets either the background color or the foreground color, but forgets to set the other one to a contrasting color. If my theme doesn't match the default assumption of black on white, it's usually the input fields that become unreadable.