If you are creating a new skin or container for your DNN site and have applied it to a page in some cases you may find it seems not to be referencing the skin.css or container.css stylesheet files. This is probably because you applied the skin or container before you created the stylesheet.
To resolve this issue go to Host > Host settings and restart the application. You will find DNN now does include the container.css and/or skin.css file that was missing before.