Avoid changing non-theme-preview requests

[okmttdhr]

What?

This PR prevents Block Theme Previews from changing non-theme-preview requests.

Why?

The current implementation would empty the original stylesheet when "there is a URL query (?theme_preview=) but no theme preview permission" (this behavior was introduced in #50338).
This unintentionally breaks functionality other than Block Theme Previews, so it has been modified not to change it except when Block Theme Previews should be applied.

How?

• Return the original value when Block Theme Previews should not be applied.

Testing Instructions

• Open the Site Editor and add the following to your URL: ?theme_preview=[themePath] where themePath is the relative path to the theme you want to preview (e.g. twentytwentythree).

[draganescu]

Yes, good catch, there should be no effect in trying to preview without permission.

[ramonjd]

Hello! Do you folks think theme previews will be part of WordPress 6.3? I'll add it to the backport issue to be sure. If not, we can remove it.

Thanks!

cc @scruffian

[draganescu]

@ramonjd if there is no major blocker appearing in the mean time I think so yes!

[scruffian]

Yes I agree, I hope it will be in 6.3.

[ramonjd]

Thanks for the feedback!