Add the following to the top of your your CSS page
@import url("/load.php?lang=en&mode=articles&articles=u:heliosanctus:MediaWiki:ChatSkin_Rune-writing_by_the_sea.css&only=styles");
Or, you may copy the code from MediaWiki:ChatSkin Rune-writing by the sea.css to your CSS page to use this Chat Skin.