State more clearly that an active theme can't be deleted

[tobifjellner]

A post in the forums ​https://wordpress.org/support/topic/theres-no-delete-button/ by @dmcohen pointed out a thing that should be better shown:

In wp-admnin/themes.php when a user with capability delete/install themes (i.e. usually an administrator) hits the "Theme details" button that is visible on hover and opens the theme details pop-up, they'll see the "Delete theme" link in the pop-up. But for the currently active theme (and its parent theme, where applicable) that link is "missing".

Suggestion: Instead of nothing, show a short message "This theme is currently active and therefore cannot be deleted."

[SergeyBiryukov]

Related: #51832

[Presskopp]

It is already shown if a theme is the "Current Theme" so in my opinion this is not neccesary

[Edit] Oh, but in case it's a parent theme nothing is shown. This could indeed be improved

[tobifjellner]

It is already shown if a theme is the "Current Theme" so in my opinion this is not neccesary

Still: Still, we'd get less confusion if we in clear mentioned that "this theme can't be deleted right now, since it's currently active/currently used as parent theme".