Settings, General, Appearance, Theme with options: System, Light, Dark

1. Rename Enable dark theme to Style
2. Change the enable/disable toggle to a dropdown with the options: System, Light theme, Dark theme
3. Set the default value for the dropdown to System; the default style of Jami will respect the user's system's configuration. This would also be consistent with the default value for Jami's User interface language which is System.

Inspiration from Ubuntu as shown in the image below:

What do you think?

Thank you