Important information regarding the template licenses:

All ExtJS templates under templates \default\_backend are depending on whether your Shopware version is licensed under AGPL v3. or under proprietary license.
If you want to modify  these templates, you have to license your modifications under AGPL v3 again.
If you use the plugin system of Shopware, you can publish your own backend modules based on ExtJS and also module modifications by purchasing the Shopware SDK under a proprietary license.

All frontend templates including _default/frontend are licensed under New BSD, so you can publish your own templates and derivations under any license.