[Improvement] Reorganize document (#1066)

docs/markdown/theme.en-US.md

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+## Custom Theme
+
+`Vant` provides a set of default themes, if you want to custom the theme color or other styles, you can use the following methods:
+
+### Option 1. PostCSS Plugin
+You can import the postcss source code in your own project, then use [postcss-theme-variables](https://www.npmjs.com/package/postcss-theme-variables) to replace the postcss variable.
+
+```javascript
+// import base style
+import 'vant/packages/vant-css/src/base.css';
+
+// import component style
+import 'vant/packages/vant-css/src/button.css';
+import 'vant/packages/vant-css/src/checkbox.css';
+```
+
+Then require the plugin in the postcss.config.js, and configure the variables according to project needs, you can view all the available variables in [profile](https://github.com/youzan/vant/blob/dev/packages/vant-css/src/common/var.css).
+
+```javascript
+module.exports = {
+  plugins: [
+    require('postcss-easy-import')({
+      extensions: ['pcss', 'css']
+    }),
+    require('postcss-theme-variables')({
+      vars: {
+        red: '#F60',
+        gray: '#CCC',
+        blue: '#03A9F4'
+      },
+      prefix: '$'
+    }),
+    require('precss')(),
+    require('postcss-calc')(),
+    require('autoprefixer')({
+      browsers: ['Android >= 4.0', 'iOS >= 7']
+    })
+  ]
+};
+```
+
+### Option 2. Local build
+Vant also support local build to custom themes.
+
+```bash
+# Clone the repository
+git clone git@github.com:youzan/vant.git
+cd packages/vant-css
+```
+
+In the local vant-css repository, modify the variables in src/common/var.css, then execute the following build command to generate the style file.
+```bash
+npm run build
+```