The quick and ugly way is to check the current customer's group and then set the theme programatically:

Mage::getDesign()->setArea('frontend')
    ->setPackageName('your_package')
    ->setTheme('your_theme');

But this solution definitely misses some flexibility.

The more sophisticated way would be to create a customer group handlers and then configure custom themes there. This solution is inspired by this article by Atwix.

So first you have to observe controller_action_layout_load_before event:

<events>
    <controller_action_layout_load_before>
        <observers>
            <customer_group_handle>
                <class>module/observer</class>
                <method>addCustomerGroupHandle</method>
            </customer_group_handle>
        </observers>
    </controller_action_layout_load_before>
</events>

Then in your observer class implement addCustomerGroupHandle method:

public function addCustomerGroupHandle(Varien_Event_Observer $observer)
{
    if (Mage::helper('customer')->isLoggedIn()) {
        /** @var $update Mage_Core_Model_Layout_Update */
        $update = $observer->getEvent()->getLayout()->getUpdate();
        $groupId = Mage::helper('customer')->getCustomer()->getGroupId();
        $groupName = Mage::getModel('customer/group')->load($groupId)->getCode();
        $update->addHandle('customer_group_' . str_replace(' ', '_', strtolower($groupName)));
    }
 
    return $this;
}

Note: The str_replace here is not foolproof so I suggest replacing it with a regular expression which will replace all non-alphanumeric characters with underscore.

So now you can set a custom theme for any customer group through xml:

<?xml version="1.0" encoding="UTF-8"?>
<layout>
    <customer_group_wholesale>
        <reference name=”root”>
            <action method=”setTheme”><theme>modern</theme></action>
        </reference>
    </customer_group_wholesale>
</layout>

The quick and ugly way is to check the current customer's group and then set the theme programatically:

Mage::getDesign()->setArea('frontend')
    ->setPackageName('your_package')
    ->setTheme('your_theme');

But this solution definitely misses some flexibility.

The more sophisticated way would be to create a customer group layout handels and then set custom themes there. This solution is inspired by this article by Atwix.

So first you have to observe controller_action_layout_load_before event:

<events>
    <controller_action_layout_load_before>
        <observers>
            <customer_group_handle>
                <class>module/observer</class>
                <method>addCustomerGroupHandle</method>
            </customer_group_handle>
        </observers>
    </controller_action_layout_load_before>
</events>

Then in your observer class implement addCustomerGroupHandle method:

public function addCustomerGroupHandle(Varien_Event_Observer $observer)
{
    if (Mage::helper('customer')->isLoggedIn()) {
        /** @var $update Mage_Core_Model_Layout_Update */
        $update = $observer->getEvent()->getLayout()->getUpdate();
        $groupId = Mage::helper('customer')->getCustomer()->getGroupId();
        $groupName = Mage::getModel('customer/group')->load($groupId)->getCode();
        $update->addHandle('customer_group_' . str_replace(' ', '_', strtolower($groupName)));
    }
 
    return $this;
}

Note: The str_replace here is not foolproof so I suggest replacing it with a regular expression which will replace all non-alphanumeric characters with underscores and then trim leading and trailing underscores.

So now you can set a custom theme for any customer group through xml:

<?xml version="1.0" encoding="UTF-8"?>
<layout>
    <customer_group_wholesale>
        <reference name=”root”>
            <action method=”setTheme”><theme>modern</theme></action>
        </reference>
    </customer_group_wholesale>
</layout>

Sure, you can check the current customer's group and then set the theme programatically:

Mage::getDesign()->setArea('frontend')
    ->setPackageName('your_package')
    ->setTheme('your_theme');

But this solution definitely misses some flexibility.

The quick and ugly way is to check the current customer's group and then set the theme programatically:

Mage::getDesign()->setArea('frontend')
    ->setPackageName('your_package')
    ->setTheme('your_theme');

But this solution definitely misses some flexibility.

The more sophisticated way would be to create a customer group handlers and then configure custom themes there. This solution is inspired by this article by Atwix.

So first you have to observe controller_action_layout_load_before event:

<events>
    <controller_action_layout_load_before>
        <observers>
            <customer_group_handle>
                <class>module/observer</class>
                <method>addCustomerGroupHandle</method>
            </customer_group_handle>
        </observers>
    </controller_action_layout_load_before>
</events>

Then in your observer class implement addCustomerGroupHandle method:

public function addCustomerGroupHandle(Varien_Event_Observer $observer)
{
    if (Mage::helper('customer')->isLoggedIn()) {
        /** @var $update Mage_Core_Model_Layout_Update */
        $update = $observer->getEvent()->getLayout()->getUpdate();
        $groupId = Mage::helper('customer')->getCustomer()->getGroupId();
        $groupName = Mage::getModel('customer/group')->load($groupId)->getCode();
        $update->addHandle('customer_group_' . str_replace(' ', '_', strtolower($groupName)));
    }
 
    return $this;
}

Note: The str_replace here is not foolproof so I suggest replacing it with a regular expression which will replace all non-alphanumeric characters with underscore.

So now you can set a custom theme for any customer group through xml:

<?xml version="1.0" encoding="UTF-8"?>
<layout>
    <customer_group_wholesale>
        <reference name=”root”>
            <action method=”setTheme”><theme>modern</theme></action>
        </reference>
    </customer_group_wholesale>
</layout>