There are several ways you can customize the way Bellows looks.

The Customizer

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.

Custom CSS through the Control Panel

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.

A Custom Stylesheet or Skin

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