feature: allow to set the selected style color

[ziemkowski]

Bug description

The day_selected style has a hard coded value of white.

It should use an easy-to override CSS variable, in case the user wants to use --rdp-accent-color: white; or some other bright value (such as yellow) on a dark-themed page.

To reproduce

Fork this CodeSandbox: https://codesandbox.io/s/react-day-picker-8-vl4ry with the code to reproduce the issue.

The above link fails to load for me (JS error in the console).

Steps

1. View the source code.

Expected behavior

I would expect to be able to define a var such as --rdp-accent-text-color: black

Screenshots

Additional context

While I'd never use such a color combination, our users can customize their page colors and we've seen plenty of users pick surprising color combinations and I expect us to receive bug reports if we don't use their primary button colors for the selected date.