this link to the real animation, not just a still image. A huerotate was animated so the spectrum fill seems moving.
The image presented here was drawn previously, with clipping, grouping, cloning etc. As somehow chrome doesn't composite clipping right, had to eliminate those for the animation and ended up redrawing the whole thing in a different z-order. Here is a video of the issue.
So the original had a z order of top to bottom like black stroke, gradient fill, black stroke, gradient fill. By that, these outlines can be switched off and their widths adjusted.
The animated one has two layers of gradient fills atop eachother with the black outline atop in a third layer.
A slightly larger version of the still image is available here.
Edit: made some improvements, this version looks closer to steam circulating around.