Feature Request: multiple `mpl_base_style`

[donbing]

Is your feature request related to a problem? Please describe. I want to apply multiple overriding styles to my plot

Describe the solution you'd like mpl_base_style should take a collection of style names

Describe alternatives you've considered converting my overriding styles to rcparams, but not all style keys appear to be valid rcparams

Additional context

[donbing]

I've added a PR for this as it seems to be very simple