Enable theming of Input component inside Dropdown component.

Related to #877 and #920 and commit #d0a9032

components/dropdown/Dropdown.js

@@ -202,6 +202,8 @@ const factory = (Input) => {
             readOnly
             ref={node => { this.inputNode = node && node.getWrappedInstance(); }}
             type={template && selected ? 'hidden' : null}
+            theme={theme}
+            themeNamespace="input"
             value={selected && selected.label ? selected.label : ''}
           />
         {template && selected ? this.renderTemplateValue(selected) : null}

components/dropdown/readme.md

@@ -55,6 +55,8 @@ If you want to provide a theme via context, the component key is `RTDropdown`.
 
 ## Theming
 
+This component uses an `Input` under the covers. The theme object is passed down namespaced under `input` keyword. This means you can use the same theme classNames from `Input` component but namespaced with `input`. For example, to style the label you have to use `inputLabel` className.
+
 | Name            | Description|
 |:----------------|:-----------|
 | `active`        | Added to the root element when the dropdown is active.|