Is it possible to hide buttons at the bottom of the screen?

Is there any method to hide three buttons at the bottom of the screen?
Can I change the background color behind these buttons?

You can set the property Visible to false to hide the buttons. If the buttons are in an arrangement, you can set the backgound color of that arrangement.

I've asked about buttons that created by Android system

Check this extension:
App Inventor Extensions: Tools | Pura Vida Apps



HideSystemUI works but expands screen vertically.
How can I avoid this effect?

At the begining of my app I calculate STEP value and draw a grid with this step.
Without HideSystemUI height and width of each cell are equal.
With HideSystemUI height is longer than width.

If you are using, for example, a VerticalArrangement (with the height set to Fill Parent) to place your grid the height of the arrangement changes when you hide the system buttons from the bottom

Take a look to the post Anke has shared.