Derive highlight color from foreground knob color

[ColonelClink]

https://github.com/ashish157/QtKnobs/blob/fd02a9862c0f35220a1802071f4d7c74f993b7d4/QtKnobs/styles/needlestyle.cpp#L96-L99

I am attempting to use a darker theme for the knobs, but the highlight color is hard coded to white, which presents problems with several colors. Instead I would prefer it be set to be relative to the foreground color of the knob: 35-50% lighter and 20-30% desaturated. This should dramatically improve the appearance of dark theming.

I would submit a pull request but I am not yet familiar enough with Qt (or C++, I am a Java programmer). How would you go about doing this? I may be able to figure it out with some guidance.