In Building a Theme on Concrete Bedrock we explored how to install Bedrock from source and integrate it into a completely new theme. That theme doesn't have a lot of personally, however. Let's take the next step: in this section, we'll modify this theme we made to look a lot different than stock Bedrock. Once that's complete, we'll create another variant of the same theme – which we call a Skin in Concrete CMS 9 - and we'll ship a second skin within the same theme. Administrators will be able to choose from either of these skins when activating our theme on the site, and can even swap between skins on different pages.
We're going to generally follow the Bootstrap 5 customization approach when customizing Bedrock for our theme. That means we're going to include Bedrock as is, but define default SASS variables ahead of time that override the Bedrock defaults and deliver a customized theme experience.