[new feature] NumberKeyboard add custom theme (#472)

docs/markdown/en-US/number-keyboard.md

@@ -9,7 +9,7 @@ Vue.use(NumberKeyboard);
 
 ### Usage
 
-#### Basic Usage
+#### Default Style
 
 ```html
 <van-button @touchstart.native.stop="showKeyboard = true">
@@ -45,11 +45,26 @@ export default {
 }
 ```
 
+#### Custom Style
+
+```html
+<van-number-keyboard
+  theme="custom"
+  extraKey="."
+  :show="showKeyboard"
+  closeButtonText="Close"
+  @blur="showKeyboard = false"
+  @input="onInput"
+  @delete="onDelete"
+/>
+```
+
 ### API
 
 | Attribute | Description | Type | Default | Accepted Values |
 |-----------|-----------|-----------|-------------|-------------|
 | show | Whether to show keyboard | `Boolean` | - | - |
+| theme | Keyboard theme | `String` | `Default` | `Custom` |
 | title | Keyboard title | `String` | - | - |
 | zIndex | Keyboard z-index | `Number` | `100` | - |
 | extraKey | Content of bottom left key | `String` | `''` | - |