Support dark mode?

[DirtYiCE]

Since 2021.6, there is support for custom dark themes: https://www.home-assistant.io/blog/2021/06/02/release-20216/#support-for-custom-dark-mode-themes
Could this theme pack gain support for this? There would be a few advantages:

• dark themes inherit from the built-in dark theme, so if there is a new element added in a new version, it would have colors that at least somewhat matches the current theme
• it would be possible to automatically switch between dark and light theme variants, where the OS/browser supports it
• some custom integrations/lovelace cards try to detect dark theme and use different colors. Right now, they think all themes in this pack is light. Like this: https://github.com/AitorDB/home-assistant-sun-card, right now I have to manually set darkMode: true if I don't want black text on dark background, but that of course won't work if I need both dark and light theme support.

A minimal implementation would be wrapping all dark themes into modes: and dark:, this would fix the first and last item from my list, but it would create an annoying dark/light switcher in the UI, and if you select light, it will of course break. A proper implementation would be to have dark/light pairs, but right now there are 3 dark variants and only one light, so the single light one would have to be added to every dark one (yaml anchors/aliases can help here to avoid copy-pasting everything).