For some time now I have used a little extension called "Improved t3skin" (extension key: t3skin_improved). It is intended to be installed alongside the default "TYPO3 skin" (extension key: t3skin). Improved t3skin makes some changes in the look and behavior of the default skin. Following is an excerpt from its manual that summarizes those changes:

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• changed highlighting colour for active and roll-over module in module list;
• changed highlighting colour for active, added highlighting roll-over colour (TYPO3 4.0.x) for pages in PageTree and files/folder in FileList;
• added highlighting roll-over colour for a lot of places. Here are some:
  • List module: highlighting of single row +  highlighting of selected button,
  • Access module: highlighting of single row + highlighting of selected button,
  • Click menu: highlighting of menu element,
  • Extension manager: highlighting of single active row,
• improved contrast (even comparing with t3skin 0.0.2);
• added highlighting of input fields at BE login page (doesn't work in IE);
• improved a little usability of lists (List module, Access module, etc) by adding some extra space around rows;
• improved usability of editing some field in multiple records (List module).

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I like the changes it makes to the skin, particularly the roll-over highlighting in the List Module and other places. Although these changes are not drastic, I do find they make the back end feel snappier and easier to navigate.