godot/scene/theme/icons/indeterminate_disabled.svg

<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><rect width="14" height="14" x="1" y="1" fill="#fff" fill-opacity=".37" rx="2.333"/><path fill="#1a1a1a" fill-opacity=".5" d="m3 7h10v2h-10z"/></svg>