Wikipedia itself is also working on a new UI: https://www.mediawiki.org/wiki/Reading/Web/Desktop_Improveme... The English Wikipedia isn't one of the "early adopter wikis" but if you visit, say, the French-language Wikipedia you can see the changes (e.g., different width to improve readability).
Every time I get to a French article somehow, I'm reminded that I should be dreading the day this gets merged into master. It's like this half-baked cards feature: whenever you search on the page and hit a link, that card will stay open indefinitely. You can't hit escape to close it, you can't hit tab to move off the link because then you'll just get another card for the next link, you have to go for the mouse. It was supposed to be an onhover feature for when you're using the mouse in the first place, instead it obscures text all the time. Luckily, cards are something you can turn off; with the narrow new style I doubt we'll have such luck. Language switching also became a mouse-requiring operation in the new design (or much more involved), other than that and the squished text I don't actually see any attempt at making it look modern.