Better customizability/themes for planner

There is a big issue with visibility of task boxes on dark theme. 
The contrast of the box edge with dark background is very low. it very hard to discern the task box edges.

There is practically no customizability for how things look in planner. all there is, is an option to change the banner color which is not very helpful.

If there was an option to change the default color of boxes that would help immensely on visibility.