I’d like to see an editor preference setting so that I can use a monospace font while editing anote but a proportional font while viewing a note. I’ve seen the entry in this forum that explains how to update the standard theme to change the font; but this is extremely user-unfriendly, especially since it needs to be enabled in every vault—and I’d want it in every vault I use.
For reference here is the css-snippet to use monospace font in source mode.
To add css-customization to different vaults, you will always need to do it separately for each vault. Usually you don’t spend much time for customization so it doesn’t require much effort to customize each vault separately. If I understand correctly vault settings can be directly copied to other vaults.
About your feature request: add new GUI setting in Settings/Appearance to set separate font for source mode / live preview / reading view. I think this addition would be well justified given that we already have other font-related GUI settings like font used in code blocks.