set $min-contrast-ratio

packages/core/scss/bootstrap/theming/_variables.scss

@@ -14,3 +14,7 @@ $gray-600: #6c757d !default;
 $gray-700: #495057 !default;
 $gray-800: #343a40 !default;
 $gray-900: #212529 !default;
+
+// The contrast ratio to reach against white, to determine if color changes from "light" to "dark". Acceptable values for WCAG 2.0 are 3, 4.5 and 7.
+// See https://www.w3.org/TR/WCAG20/#visual-audio-contrast-contrast
+$min-contrast-ratio: 3 !default;