Create extra token to set the default theme

When you set "Define css", you have some tokens. When you use the %t (path_to_theme()), it uses the path to the current theme. Sometimes you have an admin-theme selected and you want to use a stylesheet defined in the frontend-theme. In that case it would be nice to have an extra token.
I've created a patch that provides this functionality. There is an extra token available (%dt) that's representing the path to the default theme.