- Added dark mode mode constants, config storage as an integer, and a helper to resolve Auto/Dark/Light using system preferences where available.

- Replaced the dark mode checkbox with an Auto/Dark/Light color mode selector and ensured palette edits use the resolved mode.
- Applied the resolved color mode consistently across palette saving and GTK styling in the user list and channel tree/theme application paths.

src/common/zoitechat.h

@@ -83,6 +83,10 @@ gboolean zoitechat_import_theme (const char *path, GError **error);
 #define USERNAMELEN 10
 #define HIDDEN_CHAR	8			/* invisible character for xtext */
 
+#define ZOITECHAT_DARK_MODE_AUTO 0
+#define ZOITECHAT_DARK_MODE_DARK 1
+#define ZOITECHAT_DARK_MODE_LIGHT 2
+
 struct nbexec
 {
 	int myfd;