Modify theme code
You use the code editing options available within the Theme tab to modify the theme's CSS.
Customize theme bootstrap variables
Your storefront Theme CSS is made up of Bootstrap Less files and widget-specific Less files. You can use the Style Variables option within the Theme CSS to customize, and add to, a theme’s bootstrap style variables. For example, if you would like to change your theme’s viewport sizes, you can copy the Bootstrap variable from your Bootstrap documentation and paste it to the end of the Style Variables file, for instance:
@screen-sm-min: 720px
.
See Configure a widget’s style for more information.