Use the postcss-custom-properties config to define fallback colors (W…

…ordPress#24408)

Co-authored-by: Riad Benguella <benguella@gmail.com>

packages/base-styles/_mixins.scss

@@ -677,9 +677,3 @@
 	}
 	/* stylelint-enable function-comma-space-after */
 }
-
-// It is important to include these styles in all built stylesheets.
-// This allows to CSS variables post CSS plugin to generate fallbacks.
-:root {
-	@include admin-scheme(#007cba);
-}

packages/postcss-plugins-preset/lib/index.js

@@ -1,4 +1,14 @@
 module.exports = [
-	require( 'postcss-custom-properties' )(),
+	require( 'postcss-custom-properties' )( {
+		importFrom: [
+			{
+				customProperties: {
+					'--wp-admin-theme-color': '#007cba',
+					'--wp-admin-theme-color-darker-10': '#006ba1',
+					'--wp-admin-theme-color-darker-20': '#005a87',
+				},
+			},
+		],
+	} ),
 	require( 'autoprefixer' )( { grid: true } ),
 ];