I'm trying to add two custom css stylesheets just before the closing </head>
without breaking skin and/or hacking skin?
I built two css files to turn site into responsive design without actually touching any of the skin/template code so it's necessary to have in last ordering of css files.
Your best best is to register it using the DNN CSS Include skin object, adding it to your skin and simply setting a priority that is high.
<dnn:DnnCssInclude runat="server" FilePath="~/Your-PathHere" Priority="100" />
You just need to be sure that you have the following register in your skin as well.
<%@ Register TagPrefix="dnn" Namespace="DotNetNuke.Web.Client.ClientResourceManagement" Assembly="DotNetNuke.Web.Client" %>
The full API Details for Client Resource Management will help. Highest DNN priority appears to be 35