All buttons in PowerApps come by default with slightly rounded edges or corners. One simple trick to change that default look is to have completely square buttons or circular buttons. The trick is to edit the border radius of the button.
Square
In the properties of the button change the border radius to 0.
Circular
Make sure that the width and height are the same size, then in the properties of the button change the border radius to at least half of the size and width of the button.
My SharePoint Questions 