I am trying to modify the stroke and fill of an .svg image.
var s = document.getElementById("svg_img");
<img id="svg_img" src="image.svg">
If you use an img tag then for security reasons you will not be able to access the DOM of the image data. The image is effectively the same as an animated bitmap as far as the container is concerned.
<object> tag will allow you to manipulate the embedded object's DOM.