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 My page => Style section. Then click on Page, under the Colors section, as shown below.
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.
Still from the Style section, click on Image, under Wallpaper, as shown below:
Then click Reset, a shown below:
