Support Light/Dark variants for all Editor Themes, and switch with the system appearance

MacDown should support light/dark mode in a similar fashion to Xcode.

1. Each Editor Theme should have a Light/Dark variant.
2. If a light editor theme is selected and the system appearance changes from light to dark, then the corresponding dark editor theme should be automatically selected.

No files or tests are named. Start by locating the editor-theme implementation and the system-appearance handling in MacDown, then determine how themes are represented and selected. Done means every editor theme has light and dark variants, and switching system appearance selects the corresponding variant when applicable.

Written by the indexing model from the issue text.