Buy a new mouse. Or edit the client. I don't think there are other ways.
Or maybe some macro, so you can press some other key to simulate middle mouse button?
I'm not sure that I have a strong argument against sketch colors - Jorb, November 2019 http://i.imgur.com/CRrirds.png?1 Join the moderated unofficial discord for the game! https://discord.gg/2TAbGj2 Purus Pasta, The Best Client