The shape of the button to customize, but this is one of the things to customize the view, App element, component and UI, in general, more flex.
This is samples
Google support materialize https://materializecss.com/getting-started.html as basic CSS and design for their web app.
When I started to use appsheet in the couple of yeas back, Appsheet UI in general looks be based on Materialize. As it looks pretty much similar.
I hope Appsheet UI will get far better with effort of Morgan and Carie and their team, but I wish to get more options to customize to the deeper extent, like picking up codes from CSS framework to build up own web site and app.