Allow separate light and dark themes while preserving automatic style detection

Feature request

Please allow style to accept an object containing separate light and dark themes:

style:
  light: /path/to/light-theme.json
  dark: /path/to/dark-theme.json

Using the object form would preserve the automatic behavior of style: auto: Glow would detect whether the terminal background is light or dark, then use the corresponding configured theme.

The values could support the same inputs accepted by the current string-valued style option, such as a built-in style name or a path to a custom JSON style.

If either key is omitted, Glow could fall back to its current built-in style for that mode. For example:

style:
  dark: /path/to/dark-theme.json

Existing string values should continue to work unchanged for backward compatibility:

style: auto

Use case

Some terminals follow the operating system and switch automatically between light and dark appearances. Users should be able to use custom Glow themes for both appearances without changing their configuration or supplying a different --style argument after every switch.