The best way to change the background and add borders to the text tiles is through style presets from the theme. Style presets have a specific setting which allow you to change the background color and add a border that can easily be applied from the Site Editor.
To perform these changes,
1. Go to the Themes dashboard
2. Highlight the specific Theme you are using and choose Edit
3. Under Style Presets, click New
5. Provide a Name for the new preset
6. Click Save.
7. Select and expand Tiles option from the list
8. Go to the TILES section and set your preferred background color and border.
9. Click Done, and Publish the theme
10. Go back to your Site Editor
11. Apply the new Style Preset on the specific tile you want to change.