wallpaper: auto light/dark mode switching

If using dynamic scheme, auto switch light/dark mode depending on wallpaper

wallpaper.fish

@@ -122,7 +122,14 @@ else
     if test -f $C_STATE/scheme/current-name.txt
         set -l variant (string match -gr 'dynamic-(.*)' (cat $C_STATE/scheme/current-name.txt))
         if test -n "$variant"
+            # Light/dark mode detection if not specified
+            if ! set -q _flag_T
+                set -l lightness (magick $state_dir/thumbnail.jpg -format '%[fx:int(mean*100)]' info:)
+                test $lightness -ge 60 && set _flag_T light || set _flag_T dark
+            end
+            # Wait for async scheme gen to finish
             wait
+            # Apply scheme
             caelestia scheme dynamic $variant $_flag_T > /dev/null
         end
     end