It is possible to change the transparenty of the background color, you can find this option by clicking the color option (for example for the group background color) and then use the most right slider. If you slide the dot fully to te top, it will be fully transparent. Any other value will result into a more slightly transparency.
As you see, the buttons are now slightly transparent (not fully, then the dot should be on top).
This way you can alter the transparency and make it even 100% transparent, but if that happens you'll see a shadow arround the buttons.
After changing the background color, you can scroll down a little and then you'll find
Box shadow which you can turn off.