Many people nowadays use dark mode and changing colours isn’t very straightforward for that. Adding a .nightMode.card to styling should alleviate this issue.
I believe this wouldn’t complicate the code too much and on will help people customise their templates.
What do you propose to put in it? The different clients have different default background colors in night mode, so we wouldn’t want to specify a background. Also, ‘night-mode’ (kebab case) is the most common CSS class naming style. Anki & AnkiMobile support it - does AnkiDroid?
If we put color then at least it would serve as a hint for where to make a change when in night mode.
It does. (Anki manual only mentions .nightMode which is why I use it all the time).