How to tell magento that I use different theme directory? -

i'm newbie magento. downloaded module created , want modify it. have problem when change theme directory. magento reads older directory. has product view.

i try edit the, let's say, abcd.xml file inside layout folder from

<reference name='product.info'>    <action method='settemplate'><template>histheme/template/catalog/product/view.phtml</template></action> </reference> <reference name='product.info.addtocart'>     <action method='settemplate'><template>histheme/template/catalog/product/view/addtocart.phtml</template></action> </reference> 

to line this

<reference name='product.info'>    <action method='settemplate'><template>default/mytheme/template/catalog/product/view.phtml</template></action> </reference> <reference name='product.info.addtocart'>    <action method='settemplate'><template>default/mytheme/template/catalog/product/view/addtocart.phtml</template></action> </reference> 

but doesn't work!! when change above, product view part blank. please, tell me should do. thank you.

the path set templates relative theme directory.
if want different theme products, can set in backend specific product.