The easiest and simplest way to change the design of a button is through the Site Style settings in the top panel of the editor:
Navigate to the Buttons tab, choose a preset and click Edit to open customization options:
Here you can adjust the font used for the button, and also work on the design for primary and secondary buttons:
By clicking on the "gear" icon like on the screenshot above, you will be able to adjust the colors for different button components. See an example below:
Once you are done with the global button style, you can go ahead and choose which design to apply in a particular block. For that, hover over the button in a block and click on its settings:
Navigate to the Style tab and choose a suitable one:
Note: choose "Primary" button for major call-to-action like "sign up" etc. Use "Secondary" button for minor call-to-action, e.g. "View portfolio". Use "Link" to show more info - for instance, "Learn more".
Decide what happens when a button is clicked by adding a link to it: