SVG is tied closely to CSS, so you can go in your CSS and create a style for your D3 graphs and do things like:

    .chartline { stroke-width: 3px; color: red; }

D3 has some convenience functions in JS for creating SVG, but you don't have to use them, you could write SVG just like HTML, it will look more like this in the DOM:

    <g fill="none" stroke="steelblue" stroke-linejoin="round">
      ...
    </g>