Add ability to style the whole popup from outside

[dolgachio]

Description

Right now there are several class names you can pass inside of the color picker.
However, you can't style the whole popup.

My idea is to add additional classes as properties for styling purposes.

If maintainers like this idea, I would gladly submit a pull request.

[dolgachio]

@sconix @magnusbakken @peebeebee Hi all, I don't want to bother your guys, but could you take a look at my issue above?
Cause I am not sure would you accept my PR if I submit one.

[sconix]

Sure PR is welcomed for additional classes, don't see any harm in that.

[dolgachio]

@sconix thanks for the response! I will prepare a pull request and let you know.
I came up with it because I used it in a project, and our design required customization, which I couldn't implement with the current version.