Disable cssnano's colormin plugin (#15348)

It produces odd rgba values which also seem to cause issues in monaco's
color parser where the scoll shadow went red for some reason.

Regression by: #15333

Author: silverwind

webpack.config.js

@@ -95,6 +95,7 @@ module.exports = {
               discardComments: {
                 removeAll: true,
               },
+              colormin: false,
             },
           ],
         },