custom color

Seconded. None of the provided colors fit my site's theme at all. This should be a basic function for any UI module. I tried modifying the code myself but changing every instance of the ugly green color's hex code to the one I wanted didn't do anything.

Yes, A custom style option should be available in module.

But drupal also providing options for overwrite and extend libraries. → so copy SASS file in you theme and make modify then compile and apply as below code.

libraries-extend:
  we_megamenu/form.we-mega-menu-frontend:<your theme name>/<your library name>

Thanks, but I asked for an easy way.

A custom style option should be available for styling according theme.