Within this area:
Design > Responsive & Mobile Design
I opened the "Smart Phones & Small Devices" rule group and chose "Transform the layout" and selected a CSS file to transform the layout under "Specific CSS file for transforming layout and design".
Upon saving that, I realized the location that the system puts in includes ~/App_Data/ as part of the location of the file. And it appears that it also adds /App_Data/ to the CSS <link> tag, but the file doesn't load with /App_Data/ as part of it's URL...
So I figured, I'd just add the CSS as a standard part of my Master Page and ditch the CSS file link in the Responsive & Mobile Design settings.
...but... There doesn't seem to be any way to remove a CSS file linked there after it has been added...
How can I get rid of that?