Looked through all the “documentation” and it is simply not possible to use the deck as a slide template within google slides. All I can do is upload the pptx, make copy and edit the content. I cannot “apply” the theme to an existing deck. For any of the 100 advertised color schemes I need to save as and upload another version in PowerPoint. Looking at the slide master in powerpoint there is nothing that can reasonably be imported into google slides. All of it is unsupported placeholders and vectors.
None of the documentations will actually point to a proper template use.