FlatStyle for Thunderbird 0.9.2.6
This gives you a style similar to MS Office XP in your menus and toolbars. It also changes the category-selector in the options window to use your system colors, instead of always being blue. And it gives you the extra options to remove the dropdown from the Get New Messages button, and to remove the dividing lines between menus. Based on FlatStyle for Firefox by Andreas Westerlind.
GMailUI 0.5 Adds some GMail inspired functionality to Thunderbird