How many programmers does it take to change a button color?
One, but in this case it took him 35+ minutes.
It wouldn't normally be nearly so bad, though, I just had to remember the steps of the process Chris had explained to me, in addition to work around some odd UI behavior in the unity editor ("why does it keep reverting my changes?!" nearly takes on existential-crisis status in this video)
Anyway, if you're interested in UI modding, this video is helpful in that you can watch someone _else_ who doesn't know what they're doing gradually figure it out.