Click on the Design tab in the panel on the right side
If you want to style this block in another way then your App Theme, enable Customize theme for this block.
This allows you to change the color, navigation bar and the background of this specific block.
You can always remove the custom block design by disabling the switch.
How can I add a background image to my app?
When you've selected the blue Home block, you can navigate to the Design tab on the right hand side. expand the Background section and click on Media.
You can then click on the space with the + icon and add your background image. There are also options to add a filter and/or an overlay to the image.