rmfegley
In X3, and versions of Platinum prior to the 2017.03 update, customization>colors would work. Like you, I wanted a darker piano roll similar to FL Studio. The specific settings to change are piano roll view>window background to a darker gray, and then piano roll view>grid rules, beat rules, and major rules to something lighter gray(s). I can post the specific colors I used, which are close, but not identical to FL Studio's PRV colors.
soens
You can change the grid screen background and the beat lines within Sonar. If you change the main color it will cause the alternating color to change with it automatically, you cannot change them independently of each other.
But...the 2017.03 update of Platinum introduced a complication (the alternating colors for sharps and flats are now part of the theme) that does require the theme editor to get working correctly. However, this has been identified as a bug and is supposed to be fixed in the next (?) update. See this post for details.
http://forum.cakewalk.com/FindPost/3582917
EDIT: Also you can see a screenshot of my dark piano roll colors in that post.
RMFEGLEY
Your Piano Roll customisation is exactly the way I want the Piano Roll View. That dark background looks great! I would really like to hear from someone from Cakewalk in relation to this. Why has the Piano Roll in Sonar been neglected for so long. There was something very cold and clinical to Sonar PRV. Not only the colours but the way of working in it has always seemed clunky to me. I heard they have made changes to it recently so I hope they are looking at making it more user friendly.