Here is another secret, themed parties provide an instant connection and commonality among guests., not to mention an instant ice breaker.
If you walk into a regular party, on the surface the most you have in common with every one else is that you all know the host. If it’s a theme party, everyone has now one more thing in common, you’re all dressed in your 80’s garb, or western duds, or whatever.
You’re more likely to walk up to a stranger and ask “OMG, where did you find that RELAX T-shirt” which starts a funny story about having to steal it back from the teenager who finds it ironically cool, not knowing they saw them in concert when you were in your teens/twenties.
It also gets people out of the everyday hang ups of life. “Am I wearing the latest/best/nicest/most appropriate thing” vs. “screw it, I’m dressed as a house elf what could go wrong” attitude.
OP, you’ve taken it on the chin here, but if you’re up for a pub crawl wearing a blanket with arms, you can’t be a stick in the mud. Hope you have fun at the parties, whatever you wear!