Sorry for commenting on a dead thread, but for anyone who used the above gtk.css and noticed red squares replacing checkboxes in dropdown menus, the solution is to open up the file, go down to the "Firefox and Thunderbird" section, and comment out all blocks that include "-gtk-icon-source". It seems that the code points to some image files to use, but can't find them, and removing the lines reverts the code to using defaults. As for replacing the images, I still haven't found out how "url" works: it's not relative to gtk.css, and it can't be set to an absolute location.