All textboxes has black bg - thats theme default. I guess the browsers inherits that. I all applications, the font color in textbox is (almost) white - expect of browsers.
I sampled the text color in browser textbox, trying to figure out where it is defined, but it wasn't in the theme config. I also tried to change many colors to affect that, but no success. Therefor, the only conclusion I got is that this is browsers bug of not using the system definition for colors.