Allow overriding default colors for miniature

[Shenato]

In my project I use a dark theme so my SVG is hard to distinguish them from the mask in the miniature.

I wanted the ability to override the config for this rect in the MiniatureMask.jsx to other colors. I could even implement it myself and put up a PR, what do you guys think?

I could also add other colors in other components to an optional prop, or just add the colors to the miniature settings in the pre-existing API and ignore other hardcoded colors/

I was thinking i just add more configuration to the miniatureProps prop in the API so i don't change the API signature too much.