Assuming you just want to add some content via a simple template file you can add the following in your theme layout/local.xml

<default>
    <reference name="content">
        <block type="core/template" name="custom.content" template="my/custom/template.phtml" />
    </reference>
</default>

Then you just need to create your template/my/custom/template.phtml file with the content you need inside it

Assuming you just want to add some content via a simple template file you can add the following in your theme app/design/frontend/default/custom_theme/layout/local.xml

<default>
    <reference name="content">
        <block type="core/template" name="custom.content" template="my/custom/template.phtml" />
    </reference>
</default>

Then you just need to create your app/design/frontend/default/custom_theme/template/my/custom/template.phtml file with the content you need inside it