如何设置D3.js svg的背景颜色？

I'm not very familiar in styling D3.js SVG's. I create a collapsible tree and will provide an option to download this tree as SVG/PDF/PNG. This works great but the background-color of the resulting files is always transparent. Is there a possibility to create the D3 SVG with a specific background-color? I used this example for my work:I'm not very familiar in styling D3.js SVG's. I