[Feature Request] Allow customize theme

[codehz]

Is your feature request related to a problem? Please describe.
I want to support dark mode for my application, and I found there is not possible to style the mctrl control

Describe the solution you'd like
Change all hard-code color to configurable color

Describe alternatives you've considered
Add native support for windows 10 dark mode

Additional context
(Just search xd2d_color_set_rgb in mctrl's source code)