How to style content in CKEditor 5

With Drupal planning to ship with CKEditor 5 which has a much improved DX and tooling on the Drupal side, it is helpful to know how to style custom content in CKEditor 5. These styles can range from minor UI improvements on your WYSISYG editor to styles essential to match your default (front-end) theme or the content you are editing.

Because the editor instance will take styling from the admin theme, a common use of this will be for users that want their editor input to better resemble how it will appear when published (aka having the editor have the same styling as the default theme).