For Julia 1.11.2 I have installed Jupyter Notebook 7.3.1 and I fail to set the JuliaMono font, to get a notebook style as I had before with version <7.
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I once opened a related issue here - not sure if it’s the same thing though 
custom.css is now a bit diferent for j.notebook of version >=7: Notebook 7 and custom.css - Notebook - Jupyter Community Forum