Hi Matthias,

If you are talking about the tabs, that is mainly controlled by the jqueryui theme. You can choose from a number of jqueryui themes and you specify a them by this setting on the StyleSheetCombiner control in layout.master

<%-- JQueryUIThemeName valid options are:
base, black-tie, blitzer, cupertino, dark-hive, dot-luv, eggplant, excite-bike, flick, hot-sneaks,
humanity, le-frog, mint-choc, overcast, pepper-grinder,
redmond, smoothness, south-street, start, sunny, swanky-purse, trontastic, ui-darkness, ui-lightness, vader
--%>
    <portal:StyleSheetCombiner ID="StyleSheetCombiner" runat="server" JQueryUIThemeName="humanity" />

If after choosing a jqeuryui theme you still need more changes you can define CSS rules to override whatever you need. If you view the source of the page you will see there are custom classes on the body element in all admin pages. Using that class as part of a CSS selector you can override the style of anything on the page.

Hope that helps,

Joe