As Luis Benitez blogged about, there is a new IBM Connections Theme which looks pretty nice.
I just installed it on our production server but of course I had to customize it to our corporate needs. Unfortunately I had no luck doing this through changing CSS style definitions as described here.
Solution was to edit the com.ibm.social.gen4.theme_220.127.116.1130627-1409.jar directly.
If you installed 18.104.22.168-IC-Multi-LO75951 (as recommended) on the IBM Next Gen Theme for IBM Connections download site you have to change the jar file in 2 directories!
Now our Connections interface looks like this: