seriously it depends on which resort and which hotel your in. For intance Borovets town is quite high therefore snow in the resort is usual. If the hotel is in the centre eg the Rila then trainers, shoe, boots are ok as you can get to the bar quickly from the hotel, If you are in the Bor or Edelweiss, you will need good waterproof boots for the ten minute trek.
Most people seem to wear jeans or combats.
First time we went I made the mistake of taking all warm jumpers for night time - and was roasted! It might be cold outside, but all the bars are SERIOUSLY heated.
For footwear, think practical - it tends to be icy or slushy underfoot, so heels might be a bit risky. I took my uggs and they were spot on.