feat(app): inherit theme

demo/documentation/components/app/enUS/index.md

@@ -4,4 +4,5 @@ App is using to set global theme and set namespace for components (detached part
 ```demo
 theme
 namespace
+inherit-theme
 ```

demo/documentation/components/app/enUS/inheritTheme.md

@@ -0,0 +1,7 @@
+# Inherit Theme
+If you don't set theme of app, the theme of app is inherited by default.
+```html
+<n-app>
+  <n-tag>Oops</n-tag>
+</n-app>
+```

packages/common/App/src/main.vue

@@ -18,6 +18,11 @@ export default {
       NApp: this
     }
   },
+  inject: {
+    NApp: {
+      default: null
+    }
+  },
   props: {
     namespace: {
       type: String,
@@ -27,6 +32,15 @@ export default {
       type: String,
       default: null
     }
+  },
+  computed: {
+    synthesizedTheme () {
+      if (this.theme !== null) {
+        return this.theme
+      } else {
+        return (this.NApp && this.NApp.synthesizedTheme) || null
+      }
+    }
   }
 }
 </script>

packages/mixins/themeable.js

@@ -10,7 +10,7 @@ export default {
       if (this.theme !== null) {
         return this.theme
       } else {
-        return (this.NApp && this.NApp.theme) || null
+        return (this.NApp && this.NApp.synthesizedTheme) || null
       }
     }
   }