By setting the image overlay opacity at a higher percent, you will get a darker background.
Setting the image overlay opacity at a low value will generate a lighter background.
The opacity at 95%
The opacity at 50%.
This procedure is applicable in both themes 2.0 and 3.0.
STEP 1. Under Sales Channels, select Online Store.
STEP 2. On the right portion of the page, click on Customize.
STEP 3. Located on the bottom part of the sidebar menu, tap Theme settings.
STEP 4. Tap Colors.
STEP 5. To alter the image opacity, move the slider to your preferred opacity percentage.
STEP 6. Tap Save to keep your changes.