Use same (combined) icon for both color schemes

… to make the color mode switcher more perceptible

As discussed in ponylang/ponylang-website#979 (comment)

Co-authored-by: SeanTAllen <sean@seantallen.com>

mkdocs.yml

@@ -61,15 +61,15 @@ theme:
       primary: brown
       accent: amber
       toggle:
-        icon: material/toggle-switch-off-outline
+        icon: material/brightness-4
         name: Switch to dark mode
 
     # Dark mode
     - scheme: slate
       primary: brown
       accent: amber
       toggle:
-        icon: material/toggle-switch
+        icon: material/brightness-4
         name: Switch to light mode
 
 nav: