Cool party themes for adults can be centered around Rubik's Cubes, decade fashion, the seven deadly sins, custom superheroes and childhood memories. If a party is being thrown for a specific person, a good theme choice ties into the guest of honor's interests.
In a Rubik's Cube-themed party, everyone wears clothing items of different colors. People then swap clothing items with each other. The goal for each person is to have an outfit composed of one color.
There are multiple ways to throw a decades-themed party. The party could be dedicated to one decade, with everyone encouraged to show up wearing styles from that decade. Another way is to allow everyone to dress in the style of whichever decade they choose.
In a seven deadly sins-themed party, each room is dedicated to a sin. For example, the dining room could have plenty of food and be called the gluttony room. A game room with poker being played could be the greed room.
A custom superhero-themed party features guests inventing superhero concepts and showing up to the party as that character. They should have costumes and a creative storyline.
In a childhood-themed party, games from the guests' youth are played instead of adult games. Games can range from old board games to video games to outdoor sports.