Increase Graph Contrast Anyone?

I'm assuming that you want to alter the grid in the Piano Roll View:
 
Preferences >> Customization >> Colors
Select Piano Roll View as the Color Category.
The "Grid Rules" color affects the vertical and horizontal grid for non-beat values.  (This is probably what you want to change.)
The "Beat Rules" color affects the vertical grid for beat values.
The "Major Rules" color affects the horizontal octave grid and the vertical measure grid.
 
The reason I listed all 3 rules values is I find that darkening the "Grid rules" requires that I adjust the "Beat rules" and/or "Major Rules" to be able to distinguish them.
 
Hope this helps,
Ron