Sub-themes (AKA child themes) are children of the UCSB Web Theme, with one difference: they inherit the parent theme's resources. A sub-theme can be a child of another sub-theme, and it can be branched and organized however you see fit. This is what gives sub-themes great potential, while still adhering to UC Santa Barbara's visual identity system.
It's important to understand that building a child theme in Drupal 10 is easy to do and doing so provides a lot of flexibility for customization with layouts and overall functionality. If this sounds like too much for your needs and you would just like to modify, say, an icon color, please consider a 3rd party module approved from Drupal, such as the Asset Injector module or CSS Editor module.
For Drupal 10, keep in mind that any child themes created on your Web Theme site will need to be maintained and updated by the site owners (UIC), and the files must be placed in the correct location to not be overridden by a future update.
File location: /web/themes/my-sub-theme-name
The Drupal Association has an excellent manual on how to create a subtheme. Please visit their site to learn more.