There are several ways you can customize the way Bellows looks.
You can select a variety of colors and styles through The Customizer
If you’re not familiar with CSS, this is the best way to go.
If you’d like to add a bit of CSS to tweak styles that aren’t covered in the Customizer, you can Add Custom CSS via the CSS Tweaks setting in the Control Panel.
If you want to write a lot of CSS, or even an entire skin, and understand enough PHP and CSS to do so, you can Create a Custom Stylesheet or Skin