Well the static icons and graphics mentioned in the Skinning Guide are used by DNN for Admin functions... These graphics can be replaced with your own, as long as you maintian the file names, types and locations. I have done it my self on my Admin site, there is no need to modify the source for this puropose... other look and feel attributes can be modified with CSS, look at this guide (http://www.skinningtoolkit.com) for help on how to do that.

In my opinion, significant beautification can be made by modifying the system graphics and system CSS tags, I don't recommend modification to the source other than replacing the graphics and modifying the CSS, to just get a certain look and feel for the admin area, the overhead of maintaining code at every upgrade will be too resource intensive to justify that change.

Now when I say the admin area I mean the Admin Panel that appears on the top of your page when logged in as an administrator and the Wirzards, the File Manager etc.

Example:

The content part of your site that appears to your visitors and under the Admin Panel is defined by you with a skin, DNN ships with a few default skins, but you can develop your own to give your site any look and feel that your want, even for the admin pages.

To do this you need to be logged in as Administrator and navigate to Admin > Site Settings and here under Basic Settings you will see an option to select the Portal Skin/Containers and Admin Skin/Containers, select the same skin for both Portal and Admin. Now when you add content in the Admin area you will see it as it will appear in the public area of your site.

If you want to do additional changes beyond the scope of what the graphic replacement and CSS can offer then you have to seriously consider the implications of resources it will demand, not just initially but in the future, to manage each upgrade of DNN Core that comes along and the cost justification compared to the benefits achieved for such a project.

Hope this helps