Docs: Add info for themeable optgroups (forms/selects/custom.html)

Author: MauriceG

docs/forms/selects/custom.html

@@ -277,7 +277,7 @@ <h3>Multiple selects</h3>
 
 
 	<h3>Optgroup support</h3>
-	<p>If a select menu contains <code>optgroup</code> elements, jQuery Mobile will create a divider &amp; group items based on the <code>label</code> attribute's text:</p>
+	<p>If a select menu contains <code>optgroup</code> elements, jQuery Mobile will create a divider and group items based on the <code>label</code> attribute's text. The default theme for group dividers is "b" (blue in the default theme) but can be changed with the <code>data-divider-theme</code> attribute on the select menu:</p>
 
 	<div data-role="fieldcontain">
 		<label for="select-choice-8" class="select">Shipping method:</label>