Well in Mozilla( I'm sure others are similar), you go to Edit->Preferences->Appearance->Colors. It has little color boxes that you click, to select the various colors, BUT you must check the box "Use my chosen colors, ignoring the default web page colors".

The one thing I noticed as a "glitch" in the black background, is that it can partially obscure dropdown menus on some sites that use alot of "javascript menus". But you can always switch the checkbox mentioned above, to naviagte such sites. 99% of the sites are fine.