Extension:Theme modifies skin styles, such as by making a dark version, or a green version of a skin. This sort of functionality should be in core.
Support should include:
- Skins can define bundled themes with the skin in skin.json
- Extra stylesheets can be added for the theme
- less variables used within the skin can be overridden to new values (using json?) (Related ticket: T56864 )
- Sysadmin-defined custom themes (on-wiki json variable definitions, with something sort of fancy interface or something)
According to https://etherpad.wikimedia.org/p/WikiDev16-skinning, this seems to be wanted.