There isn’t really a way to increase color contrast betwen the key lines in Key editor. Tip from Greg in this video How to Change the Contrast in the Background of the MIDI Key Editor | Club Cubase March 1st 2022 - YouTube is only to brighten not to change contrast. If you don’t have perfect vision you would like to be able to change the color individually of the darker and brighter lines wich I believe you could in older versions of Cubase. If I missed somthing please tell me how to do this. If not please consider it in future updates.
The only way I know of is to set the Editor Area Background to almost fully black (“V” less than 10). The black keys will then borrow the color of the Grid Line:
I hoped for a solution in C13 bit sadly there is none.
Maybe there is still a trick someone has found,
Cubase keeps on being fresh but some usere are getting older as their sight gets weaker.