I would like to propose the improved style for NaviCube. Here's the comparison:
Pull request was created here: https://github.com/FreeCAD/FreeCAD/pull/2076
Feel free to comment here or in PR.
Reasons behind the pull request and the changes:
This PR resizes down the cube, adjusts transparency to be more easy on the eyes and fit better with the background. Arrows and cube corners sizes were adjusted for better usability and more compact appearance. Icon now uses transparency as well.
Low visibility on white background was not improved though, sorry.
While making these changes I wasn't aware that the original one was heavy inspired by Onshape. Now it's a bit different in proportion and to me it seems better then the one Onshape has.