优化CSS

public/static/theme/css/login.less

@@ -19,7 +19,7 @@ body, html {
   display: block;
 }
 
-#bgTransition() {
+body, .login-container {
   vertical-align: middle;
   background-size: cover;
   background-repeat: no-repeat;
@@ -27,10 +27,6 @@ body, html {
   #defaTransition(1s, background-image);
 }
 
-body, .login-container {
-  #bgTransition();
-}
-
 .login-container {
   height: 100%;