Each theme is make by two main components: Colors and Wallpaper.
So to remove a theme from your account, you need to remove both.
As a first step, head over to the Style section. At the top, you will see several buttons, including Colors, and if you scroll horizontally to the end, Wallpaper.
Click on Colors, and then on Default to remove, as shown below:
If you have the UNIQUE plan, then click on Reset, as shown below:
If you have the UNIQUE plan, you also need to remove the Wallpaper. All other users don't need to.
To remove the Wallpaper, click on Wallpaper at the top, then Reset, as shown below: