Currently it is not possible to use a custom theme without hacking core
a little bit. I've actually worked on implementing this, and a lot of
work went through [1], but the important patches are still waiting for
review [2] (they're low priority mostly because not that many people are
asking for the feature). See T100896 [3] for more.
Using OOjs UI widgets with no styles at all is not necessarily a good
idea – we still need *some* styling for basic things, like having the
contents of dropdown lists actually hide while they are not open, etc.
OOjs UI includes a theme called "Blank" which kind of does that, but
MediaWiki doesn't currently make it available. I'm not sure if it is
actually complete enough to be used in practice.
[1]
https://gerrit.wikimedia.org/r/#/q/status:merged+topic:custom-ooui-themes
[2]
https://gerrit.wikimedia.org/r/#/q/status:open+topic:custom-ooui-themes
[3]
https://phabricator.wikimedia.org/T100896
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Bartosz Dziewoński