Honor OS color scheme

[breyed]

Overview

Fika should have a mode to use a light or dark theme based on the OS setting. OSs are commonly configured to automatically switch between light and dark mode based on time of day. This setting should flow through to the reader mode of a web browser. For example, if macOS is configured to toggle automatically between light and dark mode, Safari's built-in reader automatically uses a light or dark mode accordingly.

UI suggestion

Replace the existing "Theme Color" group with a "Light Theme Color" group and "Dark Theme Color" group. Each group will have two of the existing themes. Add a new group (as the first group) called "Light/Dark Selection" with options "Auto", "Light", and "Dark". Make Auto the default.

Technical detail

To detect the current setting, use the prefers-color-scheme media query.