An easier way might be to do:
pip install jupyterthemes
Now you can choose from the following given themes and activate it like this
# list available themes
# onedork | grade3 | oceans16 | chesterish | monokai | solarizedl | solarizedd
jt -t <THEME_NAME>
In order to reset your theme to the default theme just use
jt -r
For all information and more, visit github.com/dunovank/jupyter-themes