Is your feature request related to a problem? Please describe.

The default Cassiopeia frontend template does not have a dark theme, while the backend template has a dark theme.

Describe the solution you'd like

I propose to make a dark theme for the Cassiopeia (default frontend template), just as it was previously done for the backend template.

I also suggest adding a new setting for website users (this setting will be displayed in the same place where the settings for the user name and time zone are shown):

website theme:

• system (based on browser or OS settings)
• light
• dark

Additional context

I think that the frontend template will evolve with each new version of @joomla, there is no longer any need to create a completely new template for each new major version of Joomla, so we could develop the existing template and its settings.