Today I just upgrade my Mathematica to 12.1 under Ubuntu 19.04.
Everything works fine but the font size is too small. As a result, I set the zoom level to 150%. Even though the font in the editor zone is big enough, the font size in the Context-Sensitive Autocompletion pop-up window is still too small to read.
I wonder how to increase the font size in the Context-Sensitive Autocompletion.
P.S. Resolution of my monitor is 4k Ultra HD.