Hmm, well I don't think it's quite one-click, in fact I think it would take fewer clicks to do it on-Wikipedia: click a link like this and then press Publish.
(Note: the theme here was picked at random from MediaWiki's `Manual:Gallery of user styles`, so... I disclaim any implication that it's a good theme. In fact, it must be bad - the link for this theme is to `en.m.wikipedia.org` !!!)
I hope you know that your browser can have multiple windows.
https://en.wikipedia.org/w/index.php?action=edit&title=Speci...
(Note: the theme here was picked at random from MediaWiki's `Manual:Gallery of user styles`, so... I disclaim any implication that it's a good theme. In fact, it must be bad - the link for this theme is to `en.m.wikipedia.org` !!!)
I hope you know that your browser can have multiple windows.