Ok, I see that the customizations do have an impact on the live web page, which is good, but isn't a preview of the changes also expected to the right of the customization options panel?
As in the screenshot, always the default settings are displayed, not the customizations of the custom skin, as expected.
Is this the normal behavior?

Sorry to bother you with that ticket, I think, what I'm experiencing is just a ConcreteCMS peculiarity.

I've spent a lot of time now trying to understand the new way of theme customization, unfortunately in vain.
Even when using the core Atomik theme, even basic customizations like font-family, font-size, font-weight is a pain. Changes take ages to apply, but are not displayed in the preview window, also custom css is not displayed as preview. I'm rather confused with this theme customization functionality of version 9+.

Also, I have learned that very often the !important rule must be used to successfully override theme styles.

So actually I'm sure your theme is working totally fine, it seems I just have to live with the way how ConcreteCMS 9 works.

Thank you,
Michael