Confirmed. This is broken in Cubase 15. If you want a light background in key editor, the CC lanes will become invisible. No other settings to change that.
I’ve already switched back to version 14 because of this issue. They said they’ve acknowledged the problem and will fix it in the next update, but it’s been a long time since they released a new version. They clearly don’t care about losing users.
I was just about to post about this. Looks like everyone there just wants to use the dark backgrounds and never tests the lighter ones which are easier on the eyes IMO. Fun times.