All colors used in Minerva are converted from Less variables
into CSS custom properties. A new file called CSSCustomProperties.less
is created in the skins.minerva.base.styles module to store
these custom properties and an ADR is provided on the rationale for
dropping support for browsers that don't support custom properties.
The new CSS custom properties follow Codex design token conventions
where possible (and noted when not).
Link colors are unique because their styles are defined in core,
so in that case the Less variables values are set to custom properties.
Those values are then fed back into MediaWiki core for core link
styling.
Also adds a temporary night-mode color palette under the
.skin-nightmode-1 class on the <html> element.
Bug: T356074
Change-Id: Ida1f14138f12bd3c600c264bde7b5100f9dbf4ff
