OK, I'm showing my lack of CSS skills now...
I've been doing basic layouts in tables, because I don't know any better, and it has been working fine. However, since changing to the BP Social theme, my tables are not displaying properly. In particular, it seems the top of the last column aligns with the previous column.
This is true on several Pages and does not seem to matter how many columns there are. I tried to trick it by adding another empty column, but it is not to be so easily tricked.