See, for example, how https://marktext.app does it in settings - it has dialogs for font name and font size. The average end user will fail to do it the way Joplin currently allows - by fiddling with Development tools and wringing CSS. I fail too. Especially considering that clicking those two Edit buttons in Appearance -> Show Advanced Settings opens a text editor - sometimes, before I picked vim to open a text file with it in Linux/XFCE from the GUI file manager, Joplin would just print "/usr/bin/wine" (???) in the console - with no content, rather than the current CSS template.
Edit: found [solved] Option control of appearance of sidebar & note list (font size and theme) - #2 by CalebJohn but the "Global Zoom Percentage" setting in that screen is missing in my Joplin desktop
Edit: this recipe worked: Changing non-editor font size