aennis thanks for your support!
Customizing Themes can be quite easily done using the source SCSS files that are part of the HTML Flavor of SmartAdmin, which contains all the sources that were used to produce the .NET Core files.
You can then use Gulp to compile the changed styles and copy over the generated (CSS) files to your .NET Core project (wwwroot) folder and that should allow you to change the look and feel.
Although .NET Core comes with a preset of available Theme variations, it does not natively support adding new Themes by itself, for that we would kindly ask you to purchase the HTML package which contains the source files you will need and comes with full instructions as well as a YouTube video on howto produce your new Theme color.
I hope this helps.