extension/resources/dark/checkbox-indeterminate.svg
<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
<path d="M0 2C0 0.895431 0.895431 0 2 0H14C15.1046 0 16 0.895431 16 2V14C16 15.1046 15.1046 16 14 16H2C0.895431 16 0 15.1046 0 14V2Z" fill="#424750"/>
<path d="M3 8H14V9H3V8Z" fill="#CCCCCC"/>
</svg>