If things just aren't working out between you and a particular theme, you can uninstall it from the Theme Manager (Tools Themes). Simply select it in the list and click the Uninstall button at the bottom, and then click OK when Firefox asks you to confirm. It's okay to uninstall the theme you're currently using — Firefox falls back to the default theme, and the uninstalled theme cries on a friend's shoulder — but you need to restart Firefox before the change takes effect. Just remember you can't uninstall the default theme.