I found this on another site for my exact model but from a few years ago as part of a Yosemite install. I basically ignored it thinking it was just a small part of the overall config.plist. Might this actually be all I need?
EDIT: The other site called this a boot plist . . . not sure if it's supposed to be the same thing as config.plist.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "
http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Graphics Mode</key>
<string>1366x768x32</string>
<key>Kernel Flags</key>
<string>debug=8 ahcidisk=1</string>
<key>Theme</key>
<string>DType</string>
<key>Timeout</key>
<string>10</string>
<key>EthernetBuiltIn</key>
<string>Yes</string>
</dict>
</plist>