I am on 19.04. If it is not possible to change the font only for the clock, I would like to change the font size for the entire top bar. I have tried changing "stage" in gnome-shell.css and also the TOP BAR section, but the font size remains the same in my bar. I have found other answers here that answer this question but those answers refer to older versions and do not refer to GTK, and so don't work.
I do not want to change the font of everything else as would happen by setting "large text" in accessibility, or using the scaling slider in tweak tool. I am curious if there is another way to do this besides creating a custom CSS override for the shell theme. I only want to enlarge the time (as I cannot see it), and if that is not possible, only the top bar.
for Ubuntu 19.04 default
add this line at around line number 786, Change the values as you wish..
If you are using Gnome-Classic via Vanilla-Gnome-Desktop
edit the file gnome
gnome-classic.css
from/usr/share/gnome-shell/theme/-----