resources/skin/container-base.svg

<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
    <g fill="context-fill" fill-opacity="context-fill-opacity">
        <rect width="6" height="6" x="1" y="1" rx="1" />
        <rect width="6" height="6" x="9" y="1" rx="1" />
        <rect width="6" height="6" x="1" y="9" rx="1" />
        <rect width="6" height="6" x="9" y="9" rx="1" />
    </g>
</svg>