Styling options for story player buttons

## Describe the new feature or change to an existing feature you'd like to see

A user requested in the Web Stories support forum a way to style buttons. For example, changing the color of the share, play, forward, and back buttons. Please share your feedback.

## Describe alternatives you've considered

Provide a clear and concise description of any alternative solutions or features you've considered.

## Additional context
related topic: https://wordpress.org/support/topic/how-can-i-add-custom-css-to-story-page/