The default mediawiki theme (vector) is also the default wikipedia theme (on desktop).

If you don't have an account you can add ?useskin=skinname to end of url to trigger different skins.

E.g. https://en.wikipedia.org/w/index.php?title=Main_Page&useskin... and https://en.wikipedia.org/w/index.php?title=Main_Page&useskin...