You can add a fully stretched background to your content blocks by following these steps:
1. Select the Design tool in the gray navigation bar. Under the Sitewide tab in the design options on the right-hand side of the preview, select Styles.
2. Select Content Blocks.
3. Tick the box that says Stretch block background to container edges.
4. Select Save and Publish to take your changes live!
To get a custom background:
1. Select the Design tab in the gray navigation bar. Select your Button in the preview panel.
2. Scroll down until you see Container and next to Background select Custom. Select the Pick From Library button or drag an image into the blue box.
3. Select Save and Publish to take your changes live!