User:Jdlrobson/Themes in MediaWiki

Vector is the most popular MediaWiki skin. As a result most MediaWiki instances look identical in style to Wikipedia. There have been various attempts to make skins "theme-able" so that this is not the case. Examples include Extension:Theme (which allowed theming of Vector and MonoBook), Skin:Bluelib (which allow skins to operate with slightly different colors) and OOUI (which has Apex and Wikimedia themes). Another obvious application of themes is to support dark mode.

Goals

We want the MediaWiki installer to look different from a production wiki
We want a single variables file shared by Vector 2022 and Minerva.
We want a default Minerva and Vector 2022 install to look different
It should be possible to substitute the variables file with a dark mode version.

How would a site configure a theme?

It is my belief that themes should operate by defining a theme inside LocalSettings.php. The configured theme would apply to all skins. In future, it could also be influenced by a user preference (although this is out of scope for now).

Configuration might look a little like:

$wgDefaultSkin = 'vector';
// A theme would be defined by setting a registered theme name.
 $wgSkinTheme = 'default'
// $wgSkinTheme = 'wikimedia';
// $wgSkinTheme = 'dark';
// $wgSkinTheme = 'apex';

The default theme would be called "default" and would be setup by the installer. The default skin would not use Wikimedia colors.
On Wikimedia servers, we'd use the "wikimedia" skin.
If the skin theme defined in LocalSettings.php doesn't exist the default theme would be used.
A theme would work on all skins that make use of skin variables.
A skin can tag itself as not supporting themes. In which case, the SkinTheme configuration option would not have any effect.

How would this work behind the scenes?

A theme would be installed in a similar way to skins and extensions. To install a theme, a folder would be installed inside the resources/src/mediawiki.less/ folder (in future this could be the themes/ folder). This folder would contain a mediawiki.skin.defaults file. The default MediaWiki install would have a wikimedia and default theme.

Currently, a skin can define a mediawiki.skin.variables file that extends a mediawiki.skin.defaults file. The location of the mediawiki.skin.defaults file is currently inside resources/src/mediawiki.less/, but with the proposed change, the directory searched will be influewnced by the wgSkinTheme variable.

A proof of concept demonstrating this approach is shown here: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/821309

Anything else to note?

It's important to note, that since Wikimedia code is open source, a 3rd party would still be able to use Wikimedia branded theme, unless the theme was made private, which would likely go against our values, but hopefully with a variety of well supported skins, we'd be discouraging it's usage/.