My question: How do I change the color of this area (Planmaker)?

[Formula Use]

I want to change the color of this area (see image)—the area I’ve circled—from green to a different color. Is this possible. And when I select a different cell, the color I changed won't revert to the original color.

Right now, I don't like using other office suites. I've set Softmaker as the default app on my phone and never use any other software.