How to disable the Theme Change in Firefox

If you are using the new design preview (check out here)

  1. Click Dark Reader icon.

    Dark Reader icon

  2. Click Settings button.

    Dark Reader settings

  3. Click Change browser theme button.

    Change browser theme

If you are using the old design

  1. Click Dark Reader icon.

    Dark Reader icon

  2. Select the More and switch between Custom and Default

    Switch between Custom and Default

Great, you did it!