With Firefox I added options to change the tab text color while a page loads, added italicized and a different colored text for unread tabs, a different color of read tabs, and even a different color for pages that the old Proxomitron filter throws back with error messages. I use a different color in the status bar text.
Similarly, I added a userContent.css mod that combined the Firefox Stop and Reload buttons. This mod does not work in K-Meleon.
They both depend on XUL. Basically in K-Meleon you can only use userContent.css to modify stuff in Web pages, not in the rest of the browser window.
(Not that I know of, but I'm not the right person to answer.)
are there any plans to provide such features?