Style tweaked a bit

I’ve been fixing some small issues with the current style, leading to a more or less complete rewrite of the CSS stylesheet. It should work fine now with Firefox 1.5.0.4 (and most likely older versions, too) and IE7 Beta 3 (basic layout should work in IE6 too). Thanks to Philipp for pointing me at some problem cases. The logo has been updated a bit, too, and should look a bit more fancy. As usual, report any problems in the comments.