1. Go to setting from smart page editor

2. Click on APPEARANCE tab.

3. you will get 3 option to set custom theme.

3.1 Gradient – you can use gradient background with two colors to set theme.

3.2 Solid color – Set single background color.

3.3 Background Image – You can add background image to your smart page.

4. Save changes.

5. Publish your smart page.