This will depend on the theme developer. However there are options if you have access to the contents.rdf file, which is recommended to make a copy of before changing anything in it. [https://developer.mozilla.org/en-US/docs/XUL/Tutorial/Creating_a_Skin] [https://developer.mozilla.org/en-US/docs/XUL/tabpanel] it should all be css properties that you can add to. If you have any questions about this, please try mdn's developer questions in stackoverflow.com Hope this helps!