add cssVar

packages/artplayer/src/player/cssVarMix.js

@@ -0,0 +1,15 @@
+import { def } from '../utils';
+
+export default function cssVarMix(art) {
+    const { $player } = art.template;
+
+    def(art, 'cssVar', {
+        value(key, value) {
+            if (value) {
+                return $player.style.setProperty(key, value);
+            } else {
+                return getComputedStyle($player).getPropertyValue(key);
+            }
+        },
+    });
+}