I use the SD Light theme, which looks the same – but I notice that whereas before we only had a dropdown box for Categories at the top, and we now have one for Tags as well.
On the Discourse Meta board It looks like Discourse has just been updated from version 2.7 to version 2.8, and that’s probably the cause.
… but I wish there was a notification here of updates and their effects on this board.
(@codinghorror will probably say that that’s up to the Mods.)