feat: add theme style toggle function

src/store/modules/setting.js

@@ -1,11 +1,15 @@
 export default {
   namespaced: true,
   state: {
-    isMobile: false
+    isMobile: false,
+    theme: 'dark'
   },
   mutations: {
     setDevice (state, isMobile) {
       state.isMobile = isMobile
+    },
+    setTheme (state, theme) {
+      state.theme = theme
     }
   }
 }