I am fairly new to python and have no html experience. The question has been asked and either not answered at all or not answered in enough detail for me to set the default
I would also suggest that you explore the options offered by the jupyter themer. For more modest interface changes you may be satisfied with running the syntax:
jupyter-themer [-c COLOR, --color COLOR]
[-l LAYOUT, --layout LAYOUT]
[-t TYPOGRAPHY, --typography TYPOGRAPHY]
where the options offered by themer would provide you with a less onerous way of making some changes in to the look of Jupyter Notebook. Naturally, you may still to prefer edit the .css files if the changes you want to apply are elaborate.