Your border and background colors are a part of the “Style” for your promotion, so you’ll need to edit the theme to make the changes.
1. Go to the “Styles” tab on the right. The style that you’re using is highlighted in gray.
2. Click on the pencil icon to edit the style.
3. Make sure the ‘Main’ tab is selected. Then, click the color box, or edit the hex code next to the section color you need to change.
Clicking the color block brings up a color chart so you can pick the color you want:
4. Once you have a color you’re happy with, just click “Save” at the bottom of the window.
Your border, foreground, and background colors will update as soon as you choose them. Make sure to save your changes so that these new colors stick!