Magento – local theme layout XML file other than local.xml

You are correct, and this is a valid & used approach for sites with multiple themes. There are a couple of additional (little-known, seldom-used) options which can be used for layout updates which apply to different store scopes: the store handle and the theme handle.

From the action controller superclass Mage_Core_Controller_Varien_Action (link):

public function addActionLayoutHandles()
{
    $update = $this->getLayout()->getUpdate();

    // load store handle
    $update->addHandle('STORE_'.Mage::app()->getStore()->getCode());

    // load theme handle
    $package = Mage::getSingleton('core/design_package');
    $update->addHandle(
        'THEME_'.$package->getArea().'_'.$package->getPackageName().'_'.$package->getTheme('layout')
    );

    // load action handle
    $update->addHandle(strtolower($this->getFullActionName()));

    return $this;
}

For a store with the code 'deutsch' and a theme configuration of 'default' package, 'german' theme, the following handles would apply to almost every view, as most views use them:

<STORE_deutsch>
<THEME_frontend_default_german>

By implementation these handles can be utilized in much the same way as <default />.

You have to set from admin panel

steps as per below

1) Open your admin panel.

2) Go to System >> Configuration >> Design Tab

3) change the fields as per below fields

4) Save config and run on front end side. It will definitely work.

I never seen the tag like "new_layout" in xml but one Suggesstion is that If you want to change the home page layout 1 column to 3 columns then then use below code in local.xml file.

 <cms_index_defaultindex>     
          <reference name="root">
             <action method="setTemplate"><template>page/3columns.phtml</template></action>
         </reference>
         <reference name="content">
             <block type="core/template" name="default_home_page" template="cms/default/home.phtml"/>
         </reference>
 </cms_index_defaultindex>