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.