fix(sfc/style-vars): should attach css vars while subtree changed (#2178)

* fix(cssVars): should attach css vars while `subtree` changed

fix #2177

* fix: fix test

packages/runtime-dom/src/helpers/useCssVars.ts

@@ -2,11 +2,12 @@ import {
   ComponentPublicInstance,
   getCurrentInstance,
   onMounted,
-  watchEffect,
   warn,
   VNode,
   Fragment,
-  unref
+  unref,
+  onUpdated,
+  watchEffect
 } from '@vue/runtime-core'
 import { ShapeFlags } from '@vue/shared'
 
@@ -27,11 +28,10 @@ export function useCssVars(
       ? `${instance.type.__scopeId.replace(/^data-v-/, '')}-`
       : ``
 
-  onMounted(() => {
-    watchEffect(() => {
-      setVarsOnVNode(instance.subTree, getter(instance.proxy!), prefix)
-    })
-  })
+  const setVars = () =>
+    setVarsOnVNode(instance.subTree, getter(instance.proxy!), prefix)
+  onMounted(() => watchEffect(setVars))
+  onUpdated(setVars)
 }
 
 function setVarsOnVNode(