How to set the background-color of a D3.js svg?

Just add a <rect> as the first painting order item that displays the colour you want.

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.right + margin.left)
    .attr("height", height + margin.top + margin.bottom);

svg.append("rect")
    .attr("width", "100%")
    .attr("height", "100%")
    .attr("fill", "pink");

svg.append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");