I am fairly new to python and have no html experience. The question has been asked and either not answered at all or not answered in enough detail for me to set the default
There is a much easier way to do without adding the CSS files and all the other methods suggested. But you have to do it every time you start the Jupiter notebook.
Go to inspect in your browser and click on the element selection icon and then click on the box. And at the bottom of the page, you will be seeing the styling option for CSS where you can easily change the font-size.