[Fizzlemizz]

A side note for future travelers, applying the Mixin directly would primarily be used for frames you didn't create but wanted to set a backdrop on (as p3lim pointed out earlier in the thread).

In the library, because lines are created by it, instead of applying the Mixin at

tipPrototype:AddSeparator(height, r, g, b, a)

you could inherit the template where the line is created

local function AcquireFrame(parent)
	local frame = tremove(frameHeap) or CreateFrame("Frame", nil, nil, "BackdropTemplate")

Both methods work but the second might save some code if after frame creation, you branch into different functions that also apply backdrops but possibly using different styles etc. (the template does the job of applying the mixin without extra lines of code)