How to theme each field of exposed filter from Views? I have found the theme template for exposed filter form of view.
Template name: views-exposed-form.tpl.php

Inside the template I found the $widgets array and for loop for each $widget display in form.

But it is rendered HTML widget How can I get only values array for each combo box to render output for my combo box as I want?

for instance:

I have one combo called Country:
Country has hierarchy level of list

Asia
- India
- Sri lanka
- thailand
- Indonesia


etc...

So, I would like to make my combo just like this

http://www.w3schools.com/tags/tryit.asp?filename=tryhtml_optgroup

&lt;select&gt;
  &lt;optgroup label="Swedish Cars"&gt;
    &lt;option value="volvo"&gt;Volvo&lt;/option&gt;
    &lt;option value="saab"&gt;Saab&lt;/option&gt;
  &lt;/optgroup&gt;
  &lt;optgroup label="German Cars"&gt;
    &lt;option value="mercedes"&gt;Mercedes&lt;/option&gt;
    &lt;option value="audi"&gt;Audi&lt;/option&gt;
  &lt;/optgroup&gt;
&lt;/select&gt;


This will identify the parent / child category in combo?
can we do this directly in drupal 7 Views for Expoesd Filters Combo ?
or
How can I theme myself to override its HTML?
You can try to catch it with hook_form_alter(): $form_id is views_exposed_form, and $form contains all the values.

$form['field_custom']['#options'] = array(
  'group1' =&gt; array('key11' =&gt; 'value11', 'key12' =&gt; 'value12'),
  'group2' =&gt; array('key21' =&gt; 'value21', 'key22' =&gt; 'value22' ),
);