[css mode] Prevent SCSS ': {' from being treated like a single token

Closes #4688

mode/css/css.js

@@ -748,8 +748,8 @@ CodeMirror.defineMode("css", function(config, parserConfig) {
         }
       },
       ":": function(stream) {
-        if (stream.match(/\s*\{/))
-          return [null, "{"];
+        if (stream.match(/\s*\{/, false))
+          return [null, null]
         return false;
       },
       "$": function(stream) {