[improvement] Color: update base green to wechat green (#2479)

docs/markdown/theme.en-US.md

@@ -17,7 +17,7 @@ There are some basic variables below, all available variables could be found in
 @orange: #ff976a;
 @orange-dark: #ed6a0c;
 @orange-light: #fffbe8;
-@green: #4b0;
+@green: #07c160;
 @gray: #c9c9c9;
 @gray-light: #e5e5e5;
 @gray-darker: #7d7e80;

docs/markdown/theme.zh-CN.md

@@ -21,7 +21,7 @@ Vant 使用了 [Less](http://lesscss.org/) 对样式进行预处理，并内置
 @orange: #ff976a;
 @orange-dark: #ed6a0c;
 @orange-light: #fffbe8;
-@green: #4b0;
+@green: #07c160;
 @gray: #c9c9c9;
 @gray-light: #e5e5e5;
 @gray-darker: #7d7e80;