Click on the « Blocks » tab ton add a button block.
Click on the « Content » tab to customize the height and the padding of the button. You can add a link in the field below.
Padding right is the distance between text and the right border of the button.
Padding left is the distance between text and the left border.
Button height of the button
These settings are only available for the button block.
Click on the « Style » tab to customize color, font, corner radius…
Be careful !
if you make some modifications when 
is selected, they will be available everywhere in the newsletter. It is quite useful if you want to change all the buttons color in the same time.
If you just want to make some changes in only one button, check the single button: