The best alternatives to Slack Themes are Gatsby, Themer, and PokéSlack. If these 3 options don't work for you, we've listed a few more alternatives below.
What do you think of Slack Themes?
AssemblyAI— Speech-to-Text API with superhuman accuracy
Themer takes a set of colors and generates text editor themes, terminal themes, and desktop/device wallpapers. (As well as themes for other apps like Slack and Alfred.)
11 Slack themes inspired by each of Wes Anderson's films. With the worldwide release of Wes Anderson's newest film "Asteroid City" this Friday, spend the week working in whimsical Wes Anderson style.