godot/scene/theme/icons/close.svg

<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16"><path fill="#fff" fill-opacity=".75" d="m1 3 2-2 5 5 5-5 2 2-5 5 5 5-2 2-5-5-5 5-2-2 5-5z"/></svg>