Change disabled buttons color in light mode to make it more visible (#30998)

app/javascript/styles/mastodon-light/diff.scss

@@ -48,6 +48,10 @@ html {
   }
 }
 
+.icon-button:disabled {
+  color: darken($action-button-color, 25%);
+}
+
 .account__header__bar .avatar .account__avatar {
   border-color: $white;
 }