I use Sharegate to migrate my classic (sub) sites to the modern site (collections). It would be great if we could apply theming / design to the moderns sites by selecting options that creates and runs a script in the background. Because we don't like to script this ourselves ;-)
It will set the logo / colors / default theme etc and creates a modern home page. So all modern sites look consistent. https://docs.microsoft.com/en-US/sharepoint/dev/declarative-customization/site-design-overview?branch=sitedesign