The above patch 573018 fixes this bug for WTE 2010.
The solution for WTE 2017 is very different.
The patch moves wikimedia-ui-base.less to core.git/resources/src/mediawiki.less/theme/wikimedia-ui-base.less for the time being.
Food for thought:
Theming that's done in LESS cannot be hot-swapped client-side.
- Different color themes for skins are not practical to be swapped server-side. That would result in multiplied cache requirements and a full stylesheet reload when switching themes.
- Only CSS that concerns branding, logo, website-specific style elements are practical to be placed into a server-side "theme". That would benefit mostly non-WMF operators of MediaWiki instances, though.
The patch is to demonstrate a possible solution. Discuss it and alternative solutions at:
Tue, Feb 18
The bug this causes on Timeless: T245476: 2017WTE uses 13px font, while VE uses the skin's default
Mon, Feb 17
Samples of 'Hack' from https://source-foundry.github.io/Hack/font-specimen.html and https://sourcefoundry.org/hack/
Samples of 'Fira Code' from https://github.com/tonsky/FiraCode
Sun, Feb 16
For future reference: the very hard to find reasons for removing 'Helvetica Neue' are:
T63470: Helvetica Neue cannot render some combining characters correctly
T65720: Helvetica rendering issues on Windows XP
Sat, Feb 15
The second patch fixes this also.