ClassicFox fixes problems with the appearance of the default theme in Windows Classic. Specifically, it restores menus to the classic style for Gecko 1.8.x, it restores the missing bevels (outlines) on toolbar buttons that have dropmarkers (bug #216266), and it addresses some of the issues from bug #430366. ClassicFox is customizable, and the user can pick and choose which fixes to apply. More information about what fixes are available with ClassicFox can be found on the ClassicFox homepage.
(Technical note: This extension is essentially a userChrome.css packaged into an extension format because, for some people, installing an extension is easier than locating and editing userChrome.css. For people who prefer to use userChrome.css, a copy of the CSS overrides used by this extension is available for download from the extension's homepage.)