Docs
Get the listed site’s theme, using a wpdb query so we don’t need to switch_to_blog.
Switch how link_tag works- should return the URL, rather than printing it. Create print_link_tag to insert the URL.
Send the source-site ID to the customizer, in addition to the previewed iframe.
Add a Customizer section & control for the previewed site.
Hide the URL field, since we don’t want anyone to be able to change that.
`customize_preview_init` is an action, not filter
Prefix control
Don’t return if we don’t have a source-site, this removes the control registration once you navigate away from the initial page.
Get the stylesheet link from theme_mods, taking advantage of the customizer.
Import CSS on “Save & Activate” in the customizer.

When our setting is saved, find and import the selected site’s CSS.
Explicitly “overwrite” CSS with new CSS.

In practice this shouldn’t overwrite anything, since changing a theme wipes the custom CSS. Doing this explicitly will prevent any strange affects from CSS for a different HTML structure, in case this changes in the future.
Remove source-site from the preview URL.
If the imported theme and the current theme are different, we can’t import, so the button should be disabled.
Change alternate label to more appropriate text
Use output buffering to force the active toggle to false when previewing a wordcamp style. This force-enables the “Save & Active” button, which otherwise is disabled when they use the same theme.