To add some style to the notebook, Jupyter has the option to add a custom css file. This file should be located in ~/.jupyter/custom/custom.css. If this file does not exist yet, create the directory if needed and add the stylesheet. Add the following code and refresh the notebook to see if it works.
body {
background-color: purple;
}
To make it easier to modify this file, create a link from the stylesheet to the working directory for the notebooks in order to be able to modify the stylesheet in the browser.
jitsejan@jjsvps:~/code/notebooks$ ln -s ~/.jupyter/custom/custom.css .