The forum styles sheet has been updated which means you may need to choose styles again below the last forum at bottom. See image UBBT77 is default.
You can see following series of styles. UBBT70 UBBT75 UBBT76 UBBT77
UBBT70 is the oldest we have. Normally one would have deleted the oldest when new ones are published but since we have members from the start in 1996 some which to keep the styles that they are used to. So check the various out and see which one you like best. as mentioned before UBBT77 default.