feat(style): add background-color var (#5112)

docs/markdown/theme.en-US.md

@@ -15,7 +15,7 @@ There are some basic variables below, all available variables could be found in
 @text-color: #323233;
 @border-color: #ebedf0;
 @active-color: #f2f3f5;
-@background-color: #f8f8f8;
+@background-color: #f7f8fa;
 @background-color-light: #fafafa;
 ```
 

docs/markdown/theme.zh-CN.md

@@ -19,7 +19,7 @@ Vant 使用了 [Less](http://lesscss.org/) 对样式进行预处理，并内置
 @text-color: #323233;
 @border-color: #ebedf0;
 @active-color: #f2f3f5;
-@background-color: #f8f8f8;
+@background-color: #f7f8fa;
 @background-color-light: #fafafa;
 ```
 

docs/site/mobile/App.vue

@@ -70,7 +70,7 @@ body {
   color: @text-color;
   font-family: 'PingFang SC', Helvetica, 'STHeiti STXihei', 'Microsoft YaHei', Tohoma, Arial, sans-serif;
   line-height: 1;
-  background-color: #f8f8f8;
+  background-color: #f7f8fa;
   -webkit-font-smoothing: antialiased;
 }
 

src/style/var.less

@@ -21,7 +21,7 @@
 @active-color: #f2f3f5;
 @active-opacity: .7;
 @disabled-opacity: .5;
-@background-color: #f8f8f8;
+@background-color: #f7f8fa;
 @background-color-light: #fafafa;
 
 // Padding