Just because you're no longer 10 doesn't mean you can't throw a themed party. Sure, it may not be Hello Kitty, princess, or Spongebob Squarepants-themed (although it totally could and SHOULD BE), but there are plenty more "adult" shows that still make parties more fun.
Over the last few years, people have thrown lavishly detailed Stranger Things parties, decades parties, and VSCO Girl/ Instagram baddie parties (just me?). But those don't always have all the classic goodies that really make it a themed party.