The Main Theme is an instrumental song part of the CliffSide OST and was featured in "Pilot". It plays during the aftermath of the trio's stand off with Death Itself.
It also briefly plays whenever someone claims a "personal feat" and/or simply someone doing an "eyedscreen". So far, only Waylon and Cordie are the only characters to do so.
CliffSide OST - Main Theme (Original Mix)
CliffSide OST - Main Theme (End Credits Mix)