The "appearance" button seen on Dutch wikipedia is able to change the SIZE of the font (text) and the width of the article. This is extremely nice to have, but it's not there on any English article...
It does have it but its in the settings menu instead. I believe the different language versions of Wikipedia are maintained by different groups so that's probably why.
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this settings page does not exist on desktop, only on the mobile version. If I go to en.m.wikipedia.org I can see it on desktop, but it's not on en.wikipedia.org. It also does not save settings from mobile version to desktop version...
Maybe the firefox reader button can help but it kind of messes the whole formatting of the page
Individual Wikipedias have individual designs and Individual design features.
is there no overhaul to copy ideas from one to another? Or overseer to somewhat keep a consistent theming?
Technically these kinds of things are decided by the Wikimedia Foundation but they'll usually not do things that the editing community of the local wiki doesn't want.
In 2014 the WMF forced a new software feature (Media Viewer) on all wikis and enforced this by "superprotecting" the JavaScript on the German-language Wikipedia so local admins (who at one point even blocked the Deputy Director of the WMF from editing) couldn't disable the new media viewer. The WMF doesn't really want these kinds of constitutional crises to happen again.