If you are having issues with Master pages and page layouts, see Master pages and page layouts.

ShareGate Migrate does not migrate the composed look or the theme gallery content.

This affects the following elements:

If the options exist in the destination version of your migration, you can manually create a site with the same look and feel, and copy the source site into it (to merge the content with the theme).

You can use SharePoint Designer for the color palette (as long as it is a compatible version). To do this, you need to navigate to the following in SharePoint Designer:

You should now see your file(s) after navigating to that location.

You can now copy those file(s) to the same spot in your destination site.