My betta is glass surfing and how do i stop it?

For information I have a 5 gal tank with a few real plants and a filter and heater. (my tank is constantly at 25⁰c or like 77⁰f)

My water quality is fine and was checked a day ago, my betta is relatively new to his tank and was placed there about 3 days ago.

But he will not stop glass surfing, he was doing it mostly to the back of the tank where I have now placed a aquarium background to stop it.

But he now does the same to the sides and sometimes the front, I'm getting a light for my tank today as mine didn't come with one. Will the light fix the issue or will I also need to cover the sides?

The petstore employee told me that she had painted her betta tanks back and sides with black paint, will this be a good idea?