There are two ways of doing that:
- create your theme using Create new Site dialogue
- create your theme manually
1. Create new Site dialogue
Create New Site dialogue lets you create your own theme.
To do it you need to switch to Theme tab and check Create new theme checkbox
After that your site theme will be created somewhere here:
/sitecore/media library/Project/Tenant/Site/My Site Theme
2. Create your theme manually
If you want to do it manually you need to repeat script steps.
What scaffolding script does is:
- create new Theme item
- assign theme base templates
- copy all content of Basic theme under your new Theme
- assign your new theme to allowed/compatible themes in your site settings
You can configure them here:
Without that step, your new theme will not appear in any site options, for example here (device to theme mapping)