styles: Implement mediawiki.skin.variables.less for MinervaNeue

For now, only define:

- @font-family-sans (unused in MinervaNeue, but for possible future use)
- @border-radius-base

With current implementation before this patch, MinervaNeue is relying on core's default
`@border-radius` which is set to `0` to be close to HTML default. 
MinervaNeue is currently not applying correct skin's `border-radius`.

Bug: T263270
Change-Id: I9a149eefd15efede27416ada4db07072e53628ec

dev-scripts/setup-storybook.sh

@@ -4,7 +4,12 @@ set -eu
 mkdir -p .resolve-less-imports/images
 mkdir -p .resolve-less-imports/mediawiki.ui
 
+# Copy skin's mediawiki.skin.variables.less to use it over core's own, which is removed.
+rm -f .resolve-less-imports/mediawiki.skin.variables.less
+cp resources/mediawiki.less/mediawiki.skin.variables.less .resolve-less-imports/
+
 # Fetch resources via curl, `-sSL` silently, Show only errors, Location header and also with a 3XX response code.
+curl -sSL "https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/master/resources/src/mediawiki.less/mediawiki.skin.defaults.less?format=TEXT" | base64 --decode > .resolve-less-imports/mediawiki.skin.defaults.less
 curl -sSL "https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/master/resources/src/mediawiki.less/mediawiki.skin.defaults.less?format=TEXT" | base64 --decode > .resolve-less-imports/mediawiki.skin.variables.less
 curl -sSL "https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/master/resources/src/mediawiki.less/mediawiki.mixins.less?format=TEXT" | base64 --decode > .resolve-less-imports/mediawiki.mixins.less
 curl -sSL "https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/master/resources/src/mediawiki.less/mediawiki.ui/variables.less?format=TEXT" | base64 --decode > .resolve-less-imports/mediawiki.ui/variables.less

minerva.less/minerva.variables.less

@@ -1,4 +1,5 @@
 @import 'mediawiki.ui/variables';
+@import 'mediawiki.skin.variables.less';
 
 /**
  * System font stack for sans-serif fonts

resources/mediawiki.less/mediawiki.skin.variables.less

@@ -0,0 +1,14 @@
+/**
+ * This file is a LESS @import match for 'mediawiki.skin.variables.less'
+ * when MinervaNeue is the active skin.
+ */
+
+@import 'mediawiki.skin.defaults.less';
+
+// Body content uses the user setting in browser / the system default sans-serif font:
+// Arial on Windows, Roboto on Android.
+// FIXME: Use WikimediaUI Base's OS specific default sans-serif fonts.
+@font-family-sans: sans-serif;
+
+// Matches WikimediaUI Base and OOUI/WikimediaUI theme.
+@border-radius-base: 2px;

skin.json

@@ -152,6 +152,9 @@
 			"MinervaHooks::onOutputPageBodyAttributes"
 		]
 	},
+	"SkinLessImportPaths": {
+		"minerva": "resources/mediawiki.less"
+	},
 	"MessagesDirs": {
 		"MinervaNeue": [
 			"i18n"