MatmaRex (Bartosz Dziewoski) recently rewrote the CologneBlue skin for better compatibility with modern MediaWiki standards -- see https://gerrit.wikimedia.org/r/#/q/status:merged+project:mediawiki/core+bran... . In case you're interested, here's what he learned:
"The things I learned are really mostly applicable to rewriting cologneblue again ;) (or maybe other legacy skins). One big gotcha was documented in https://gerrit.wikimedia.org/r/#/c/29422/ , another was a tiny little bug i fixed in https://gerrit.wikimedia.org/r/#/c/26819/ , last one was https://gerrit.wikimedia.org/r/#/c/26808/ - and I'd say that's it. All or most of the ugly things there were probably caused by lack of a standardised interface like the SkinTemplate, so every skin had to reimplement basic stuff like navigation. And then we have code duplication, and it all being reimplemented slightly differently in SkinTemplate, and legacy skins not supporting new hooks, because nobody cared. But that's the usual story."
Anyone else trying to rewrite old MediaWiki skins? Any lessons learned?