Hah, silly me. Didn't see that there.
Hopefully it should work now.
Edit: No, it isn't. I'll make some screenshots.
I have tried deleting the original custom file and replacing it with a new one multiple times. As well as restarting Steam.
And if you meant instead of the Steam Metro skin folder, that was the first thing I tried. I didn't try the skins folder at first, but this was the first thing I tried.
Again, at this point, I've tried having a blue custom file in both of these folders multiple times. I've opened settings multiple times. I've deleted old custom files multiple times, and I've restarted and closed out Steam multiple times.
Could the fact that I'm not using the new Steam beta have anything to do while it isn't working, or am I just not getting the instructions yet?