feat: implement read/write of custom properties

lib/CSSStyleDeclaration.js

@@ -35,6 +35,9 @@ CSSStyleDeclaration.prototype = {
    * Returns the empty string if the property has not been set.
    */
   getPropertyValue: function(name) {
+    if (name.indexOf('--') === 0 && this.hasOwnProperty(name)) {
+      return this[name];
+    }
     if (!this._values.hasOwnProperty(name)) {
       return '';
     }
@@ -56,13 +59,19 @@ CSSStyleDeclaration.prototype = {
       this.removeProperty(name);
       return;
     }
-    var lowercaseName = name.toLowerCase();
-    if (!allProperties.has(lowercaseName) && !allExtraProperties.has(lowercaseName)) {
+    var isCustomProperty = name.indexOf('--') === 0;
+    // custom properties are case-sensitive
+    var propertyName = isCustomProperty ? name : name.toLowerCase();
+    if (
+      !isCustomProperty &&
+      !allProperties.has(propertyName) &&
+      !allExtraProperties.has(propertyName)
+    ) {
       return;
     }
 
-    this[lowercaseName] = value;
-    this._importants[lowercaseName] = priority;
+    this[propertyName] = value;
+    this._importants[propertyName] = priority;
   },
   _setProperty: function(name, value, priority) {
     if (value === undefined) {