Create a configurable theme to use with the Event Platform

Problem/Motivation

Creating this issue here since it's spirit is in keeping with the goals of the Event Platform, even though it will likely be implemented as a separate project. This issue can serve as a place to collaborate on gathering requirements.

Event planners want a similar, low effort way to create a public theme for their sites based on the Event Platform.

Proposed resolution

Create a theme that has sufficient configuration options to create a custom feel without having to implement a fully custom theme. The Gin admin theme has a variety of configuration options and also can use a custom CSS file, and so can be a source of inspiration.

I am personally inclined to implement such a theme as a subtheme of Olivero, but that can be a point of discussion.

Additional configuration options could include:
- text font
- heading font
- text color
- heading color
- page background color

Some kind of accessibility checking of color combinations would be ideal, but maybe this could be better implemented using an existing tool.

Remaining tasks

User interface changes

API changes

Data model changes