First, install jupyter notebook.
Write the command in the jupyter notebook cell which is given in the folder "Download_Theme" to download the themes of jupyter notebook.
For checking the availability of themes you can type !jt -l in cell then you will see all the themes which are available in jupyter notebook.
After choosing what theme you want simply type whatever is written in Dark_Theme file of this repository with your theme name which you want in your notebook cell and reload Chrome to see the changes.
To restore the default theme of jupyter notebook you have to simply type !jt -r in the cell and reload Chrome to see the default theme of jupyter notebook.