fix(cli): duplicated site base style

packages/vant-cli/site/common/style/base.less

@@ -1,13 +1,13 @@
-@import './variable';
+@import './var';
 
 body {
   min-width: 1100px;
   margin: 0;
   overflow-x: auto;
-  color: #333;
+  color: @van-doc-black;
   font-size: 16px;
   font-family: PingFang SC, 'Helvetica Neue', Arial, sans-serif;
-  background-color: #f0f3f6;
+  background-color: @van-doc-background-color;
   -webkit-font-smoothing: antialiased;
 }
 

packages/vant-cli/site/common/style/highlight.less

@@ -1,4 +1,4 @@
-@import './variable';
+@import './var';
 
 code {
   position: relative;

packages/vant-cli/site/common/style/index.less

@@ -1,3 +0,0 @@
-@import './variable';
-@import './base';
-@import './highlight';

packages/vant-cli/site/common/style/var.less

@@ -4,6 +4,7 @@
 @van-doc-fuchsia: #a7419e;
 @van-doc-text-color: #34495e;
 @van-doc-text-light-blue: rgba(69, 90, 100, .6);
+@van-doc-background-color: #f7f8fa;
 @van-doc-grey: #999;
 @van-doc-dark-grey: #666;
 @van-doc-light-grey: #ccc;