1. Select Design in the navbar
2. Navigate to your desired page using your site's navigation
3. On the righthand side, select the "Page" tab
4. Select the block title that you're changing
5. From here, find the Container dropdown near the bottom
7. Now, select your desired color and opacity to accomplish the look you like
Note: Background colors with decreased opacity can show a custom image background underneath it.
8. Select Save and Publish to make it live