At the moment I've added the attribute "ThemedCssFolderUrl" in the Onet.xml file and it works nicely. I got 7FA80357 from saving the custom composed look as a template and looked in the onet file. But this is a custom "id".
<Configuration ID="1" Name="Default" MasterUrl="_catalogs/masterpage/seattle.master" ThemedCssFolderUrl="/_catalogs/theme/Themed/7FA80357">
But if we look at a "predefined look" the ThemedCssFolderUrl ="_themes/0" But i can't find if these potential standard design schemas in the sharepoint root.
Is there an out of the box way to make new Sub-Sites inherit the color theme from the root-site/Parent?