fix(themes): don't leak custom theme css after preview (@fehmer)

[fehmer]

Fixes #7875

Slow network connections caused the theme css to be applied on the #customTheme stylesheet link element after clearing the theme preview with href="".

Empty href is invalid, so best to remove the whole element if we don't need it:

The address of the link(s) is given by the href attribute. If the href attribute is present, then its value must be a valid non-empty URL potentially surrounded by spaces.

https://html.spec.whatwg.org/multipage/semantics.html#attr-link-href

While chrome is applying the last css file firefox removes it correctly but keeps the loaderBar active because of the missing onLoad callback

[Miodec]

Thanks