To create a new visual style, simply implement the DIMod interface and provide a DisplayableInstrument (or DefaultDisplayableInstrument) implementation. Then package it in a jar and add a file under the META-INF/services/ folder in the jar called emergencylanding.k.exst.mods.IMod, and add a line with the fully qualified name of your DIMod implementor. Then create a folder named mods in the launch directory and place the jar inside.
Note: You cannot yet change the visual style. Implementation of this is being worked on.