If you're one of the small group of users that don't use the default theme for these forums then it's possible it was reset on you. If so, I'm sorry, you can reset it in your user control panel.
I'm testing out an upgrade to the forums using another copy of the board, but during early testing it overwrote the theme settings for the live one. Thankfully, that was all it did
The existing board software is many years out of date and is no longer maintained, and we need to upgrade to a new major version in order to keep up with newer web standards, security, board features etc. The underlying code for the new version is really quite different to our current one, so it requires solid testing first, to ensure we don't lose anything and the board doesn't crash. The possible bad news is that all the old themes (including this one) are going to be lost, and I'll have to do my best to recreate them. Maybe it's time for a new board theme, anyway. Some of the board extensions may also be lost, but I'm hoping I can find compatible equivalents.