Creating Custom Textures for Addons

[Cizox]

Hello,

So recently, I got some addons. And I do not like the way that they look. Is there any way I can modify them so I can add a custom texture? You know, something like this:



See how he has a custom texture for Recount, and Omen. So is there at least some sort of tutorial on how to make custom textures for addons? Thanks.