Adding local.css into header from local.xml

I have a strange problem i think. Usually i add custom styling sheets into head with following code:

<layout version="0.1.0">
<default>
    <reference name="head">
        <action method="addItem">
            <type>skin_css</type><name>css/local.css</name>
        </action>
    </reference>
</default>
</layout>

But on custom theme i am working on, this add local.css into base package like this:

/skin/frontend/base/default/css/local.css

I want it to be like this:

/skin/frontend/default/mytheme/css/local.css

What am i missing here?