I have been using the AppImage on Linux Mint for some time (including the pre-releases up to 1.2.4), and the font size control using Joplin settings for the editor pane and userstyle.css for the viewer pane has always worked. However from the image in your GitHub issue it looks like it's the editor font that is huge as opposed to the viewer. Do you want the editor (left pane) font smaller or the viewer (right pane) font larger?

If the former, what font size did you enter in Tools > Options > Appearance > Editor font size? Try setting it to 12.

Also, have you fully shut down (File > Quit) and restarted Joplin after changing the userstyle.css file?

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