Dear mercime,
thanks a lot for your reply, I'll try using a child-theme (I already noticed that "child" matter in the description of some themes, if this might help me, perhaps?) . Anyway -
did I understand this right: child-theme on main site, and child theme on subsite? (sub-blog; didn't intend to use subdomains, so only difference would be the url: familie-stracke.net versus familie-stracke.net/amelie-rosemarie/ and so on.
Second, you do not need to activate bp-default theme for subsite/subblog unless you were planning on ... ?
What should I have planned, then? I intended that this site should become a "family-site", where each registered member of my family or friends of should be able to create their blog on its own. (So far, this should even work without Buddypress, but Buddypress looks fine, too.) However, 'look and feel' of any blog should be the same, only difference content and url.
I'll try a little, now (and then go to bed; children were a little laborius...