1. bright green on a white background is not very readable at all. for the individual environment themes I would adjust them so you maintain propper color contrast to enhance readability.
2. I am not in love with the overlay because the Z axis often overlays with the top view. Sometimes Y cannot be seen.
the idea to combine both axis and viewcube makes sense but I am not sure if it is as effective. I would rather split them apart maybe.
I might rather offer another option for just showing the axis on the object ground
Show grid
Show Axis and maybe offer a Z axis as well.
I find this a lot more informative to know where and what x and y and z do when you see it next,under, at your object