[Turtle91]

I was playing around with this in Sigil and was able to change the equation size by changing the height of the image using css. I couldn't get the color of the stroke to change using the class because it is defined in the .svg file itself using the <g style="fill:rgb(0%,0%,0%);fill-opacity:1;"> tags at the bottom. You would need to change those individually (or with a regex). eg to make them all green replace rgb(0,0,0) with rgb(0,255,0), or black:rgb(0,0,0)

You might be able to increase the size by using CSS:
.fs img {height: 1.2em}

The light grey color you see may?? be caused by the '%' in the rgb(0%,0%,0%)...your reader may be hanging up on that...I'm certainly not an expert on SVG, but I haven't seen rgb described with percentages before...I just use a given value from 0-255.