AquaCSS has some prebuilt themes. Here's a complete list for you.

Comet
Natural
Neon
Nord (Default)
Rose
Seaweed
Space
Terminal

You can also make your own theme with this.

        @import url("https://aquacss.darth-ness.repl.co/")
        :root {
        --red:#bf616a;
        --orange: #d08770;
        --yellow: #ebcb8b;
        --green: #a3be8c;
        --blue1:#8fbcbb;
        --blue2:#88c0d0;
        --blue3:#81a1c1;
        --blue4:#5e81ac;
        --purple: #b48ead;
        --default:#2e3440; 
        --default2:#3b4252;
        --default3:#434c5e; 
        --bhover:#4c566a; 
        --text:#eceff4;
        
        }]
    

The colors are the default values.
Note: If you don't want to change one of the vaules, you can just remove it.