Volto Style Customization

I create a new Volto site inside a cloned Github Volto repository. I was able to customize the logo of the Volto site. This works with a new logo inside the customizations subfolder with the structure 'components/theme/Logo/Logo.svg.
I wonder about the way to change the styles of site elements. I tried to follow the example about changing the color of the divider of the breadcrumbs from the Plone Volto training 2021: 4. Styling — Plone Training 2021 documentation
I thought that I have to create the customization inside the subfolder with this name in the src folder of my new Volto site, but I couldn't get it to work, although I tried out (nearly) all possibly options.
My Volto site is using the Pastanaga theming.

Maybe you can share the repo? The theme folder should go into the root of the Volto project. Also, try to restart Volto, if you haven't already done so, maybe it helps.

Thanks for the hint. I thought all customizations had to go into the
src/customizations subfolder of my Volto site. The correct place is the
root folder of my Volto site and the subfolder 'theme' there.

Currently my Volto site is only a test project to get familiar with the
new Volto frontend.

Did you start with your new Volto app with the following recommended way?

npm init yo @plone/volto

See Bootstrap Volto - Volto Developer Documentation

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