Go to line 313 and change:
searchBox = CreateFrame("EditBox", nil, window)
to
searchBox = CreateFrame("EditBox", nil, window, BackdropTemplateMixin and "BackdropTemplate")

Go to line 323 and change:
edgeSize = 0,
to
edgeSize = 1,

#30

Damn. You beat me to it. I got the same results by changing edgeSize to nil. I've been fixing various addons like BuckSack, XP Lite and OmniCC. 99% of the required changes are literally just adding "BackdropTemplate" to CreateFrame() for frames that use backdrops, lolz.