Jupyter lets you change the CSS of an individual notebook within the notebook itself as demonstrated in this answer:
from IPython.core.display import HTML
HTML("""
<style>
// add your CSS styling here
</style>
""")
However Jupyter makes these changes persist somehow and I can't figure out how to get rid of them short of creating a new notebook: clearing the browser cache, trying a different browser and restarting the notebook server don't help. I also can't find any file in ~/.jupyter
that looks like the culprit.