This is easy to do using the jupyter-themes package by Kyle Dunovan. You may be able to install it using conda. Otherwise, you will need to use pip.
Install it with conda:
conda install -c conda-forge jupyterthemes
or pip:
pip install jupyterthemes
You can get the list of available themes with:
jt -l
So change your theme with:
jt -t theme-name
To load a theme, finally, reload the page. The docs and source code are here.