In X2, you can select a different background colour for every track in multiples of 10; i.e. whatever colour you choose for Track 1 will be repeated on Tracks 11, 21 etc:
You can do exactly the same for the foreground (i.e. the waveform) in the same way.
In X2, my colour scheme (to aid my poor contrast eyesight) was to have dark blue as the foreground colour for all tracks, pale yellow for the background of unselected tracks, and light blue as the background for the selected track:
As far as I can tell in X3, although you can still choose the foreground colours as in X2, the background colour of unselected tracks
cannot be changed, there is only the option to choose the background colour for the
selected track:
Carrying on with my high contrast theme, I currently have the selected clip set to a rather garish red, which does seem to work OK: