First you need to precache your shader in the init() section in main.gsc.
In the case of sharks menu base (might be the same in others too), you need to find
self.menu.scroller in the storeshaders() section. Replace
"white" with the shader name...I also found that I had to replace
(0,0,0) with
(255,255,255) otherwise it wouldn't show the shader.
This is done from memory and on my phone, but Im sure you can figure it out from this