add: add transition.css

src/styles/index.scss

 @import './mixin.scss';
+@import './transition.scss';
 @import './btn.scss';
 @import './element-ui.scss';
 @import './sidebar.scss';
@@ -109,17 +110,6 @@ code {
   }
 }
 
-//globl transition css
-.fade-enter-active,
-.fade-leave-active {
-  transition: opacity 0.28s;
-}
-
-.fade-enter,
-.fade-leave-active {
-  opacity: 0;
-}
-
 //main-container全局样式
 .app-container {
   padding: 20px;

src/styles/transition.scss

+//globl transition css
+.fade-enter-active,
+.fade-leave-active {
+  transition: opacity 0.28s;
+}
+
+.fade-enter,
+.fade-leave-active {
+  opacity: 0;
+}