Add mode 2031 support to automatically report dark or light theme. tmux

will guess the theme from the background colour on terminals which do
not themselves support the escape sequence. Written by Jonathan
Slenders, GitHub issue 4353.

input-keys.c

@@ -314,6 +314,12 @@ static struct input_key_entry input_key_defaults[] = {
 	{ .key = KEYC_DC|KEYC_BUILD_MODIFIERS,
 	  .data = "\033[3;_~"
 	},
+	{ .key = KEYC_REPORT_DARK_THEME,
+	  .data = "\033[?997;1n"
+	},
+	{ .key = KEYC_REPORT_LIGHT_THEME,
+	  .data = "\033[?997;2n"
+	},
 };
 static const key_code input_key_modifiers[] = {
 	0,