Thanks for your additional input! I already did some "research" for REGEX and used ActivePerl to get the job done. I saw the info in the header of the file for REGEX which was a great help. I made a little perl script which reads a file line by line, applies
s/Regex Match/Regex Replace/g
to every line and writes the result to a new file. No big deal when you have the help system from ActivePerl and some programming experience in other languages. I can post the script if it is of interest.
Anyway, I digged a bit more with firebug and the original script and found the exact line where the kinda white background is defined. It is in \components\com_comprofiler\plugin\user\plug_cbactivity\templates\default\template.css in the block
.cbActivity .cbActivityInner .well {
The background-color is set to #f5f5f5. I changed it to #252525 and was fine with the default template. So there is no hurry in designing my own template.
It is in line 5035 according to the counting of my text editor. Maybe this helps someone else.
Kyle, thanks again for giving me a kickstart in template design!!! I made a design with stylebootstrap.info, but it is rather short compared with the original template.css. There is so much more in your css file and I have the feeling many things have no effect on my site - from a first look. I guess I just haven't seen everything that CB has to offer.