In Default Ubuntu 19.04
the User Shell Theme is
If you want to use your own shell theme, you need to install user-shell theme. https://extensions.gnome.org/extension/19/user-themes/
If you are using the default one,
open the file
/usr/share/gnome-shell/theme/Yaru/gnome-shell.css and edit the content like below. Please note that I have already added the line
font-family: Arial, Helvetica, sans-serif; in below content.
/* TOP BAR */
font-family: Arial, Helvetica, sans-serif;
font-feature-settings: "tnum"; }
for Example: In the belwo .gif I have changed the top bar font from "Keraleeyam" to "Dyuthi"