Include a base 'theme' for default editor styles

We should supply a base set of styles that can be used for when the editor is used outside of a WordPress environment, or in a shadow DOM situation.

This would only be necessary for whatever styles are provided by a theme. For example the `ul` or `ol` styles often need a lot of changes outside of WordPress.

No file or test is named. Start by locating the styles supplied by themes and checking the editor outside WordPress and inside a shadow DOM. Identify the base rules needed for theme-provided elements such as ul and ol, then verify those contexts render with the expected default styles.

Written by the indexing model from the issue text.