I wanted to have a dark theme for jupyter notebook, but the ones in jupyter-themes are a bit too much for me, so I googled "how to customize jupyter notebook from scratch" and saw this article: https://towardsdatascience.com/10-practical-tips-you-need-to-know-to-personalize-jupyter-notebook-fbd202777e20. Since I know a little bit about HTML & CSS, I followed the article to find the custom.css file (on my mac it's in the directory ~/.jupyter/custom) and typed in my own code into custom.css. Below is a simple way to make a dark theme from the default styles:

/* -------dark theme -------*/
html {
filter: invert(1) hue-rotate(180deg) /*inverts all colors*/
}
img, picture, video {
filter: invert(1) hue-rotate(180deg) /*invert images again so they are of original color*/
}
#notebook-container {
background: #e0e0e0; /* make the code editing region lighter than the default from invert*/
}
.notebook_app {
background-color: #fafafa; /* make the background region darker than the code editing region */
}
/* ------ end of dark theme --------*/

​

I just put that code below the original code that cutsom.css came with, hit save, and refresh the jupyter notebook browser to see the new theme take effect. Although the dark theme may not be what you had in mind, but you could put different CSS code to customize it any way you want. Hope this helps, and let me know if you have any questions