Unable to Change the look and feel for Office 365 groups site, stuck on the status "working on it"

I too have the same issue for "modern" team sites. It just sits there spinning on "Working on it..." for a very long time. What's strange is I was able to "change the look" about a month or two back. It was slow and it didn't feel quite right, taking perhaps 1-2 mins to "try" before allowing me to apply, but it worked.

My reading suggests the PnP Provisioning Engine may be needed to change themes for modern team sites:

https://msdn.microsoft.com/en-us/pnp_articles/modern-experience-customizations-customize-sites

The provided powershell script didn't work for me either, I get the error "the operation has timed out". This too is a known issue, with one of the contributors of the former pnp article above commenting:

"This is unfortunately known issue around theme apply process with modern sites"

Reference: https://github.com/SharePoint/PnP/issues/1622