1. Go to your report, and select design in the side menu (if the side menu is not open click on the report menu icon  to open it first).
2. Click on edit* above the report's color theme.
3. Click on the color theme you want to delete and choose select. If you want to select all the color themes, click on the select all icon ().
4. Click on the delete icon (). The selected color themes will be deleted immediately.

* This feature is only available in the PRO and PRO+ plans. Check out our plans.