Add shades of gray to the variables (beta)

packages/@uppy/core/src/_variables.scss

@@ -16,6 +16,19 @@ $color-yellow: #ffd600 !default;
 $color-green: #1bb240 !default;
 $color-blue: #2275d7 !default;
 
+// Shades of gray
+// @todo: replace all current grays with them
+$gray-50: #fafafa;
+$gray-100: #f4f4f4;
+$gray-200: #eaeaea;
+$gray-300: #dfdfdf;
+$gray-400: #bbb;
+$gray-500: #939393;
+$gray-600: #777;
+$gray-700: #525252;
+$gray-800: #333;
+$gray-900: #1f1f1f;
+
 $color-uppy-pink: #eb2177;
 
 // Sizes