Hi there,
1. Select the Button – in the settings sidebar > select Spacing.
Here you can set the border and the border-radius for the button.
2. To have full width blocks you must first make the page Full Width which you can do with the Page Builder Container meta box:
https://docs.generatepress.com/article/page-builder-container/
Now the page is full width you will need to use the Container Block to divide your page into pieces. By default the Container will have a Full Width background that you can add color to.