There is! I recommend exploring the "free play" App Lab to be able to see all of the commands and then reading the documentation too!
For this one, I wasn't sure if it was possible, so I started out in the design mode to see if I could change it in design mode, and you can! (see the last property here)
From there, I went to the "setProperty" command and read the documentation. In the documentation, it said you could set the "background-color" of an UI element. Since "background color" is the property name in design mode, I tried that out.
You can see the example app I made here. Ignore the other UI elements - I re-mixed an empty ColorSlueth App so there are some left over elements there.