Is the useSkin parameter something you manually added? I am not logged in and when I navigate to another page the parameter (and with it the skin preference) disappears.
For similarly newly enlightened people it's a feature of MediaWiki (which Wikipedia runs on) and there are five (plus two) themes selectable with ?useskin= values:
- timeless (as seen above): https://en.wikipedia.org/wiki/Wikipedia:Skin?useskin=timeles...
- monobook (default 2004-2009): https://en.wikipedia.org/wiki/Wikipedia:Skin?useskin=monoboo...
- vector (default 2010-2021): https://en.wikipedia.org/wiki/Wikipedia:Skin?useskin=vector
- vector-2022 (desktop default 2022-): https://en.wikipedia.org/wiki/Wikipedia:Skin?useskin=vector-...
- minerva (mobile default): https://en.wikipedia.org/wiki/Wikipedia:Skin?useskin=minerva
- modern (delightedly dated, deprecated): https://en.wikipedia.org/wiki/Wikipedia:Skin?useskin=modern
- cologneblue (also ancient and anti-favored): https://en.wikipedia.org/wiki/Wikipedia:Skin?useskin=cologne...
1: https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsec...