Adding the above CSS (without the <style> HTML tags) to userstyle.css changes Markdown preview and Rich Text editor fonts to Courier New. If you don't have Courier New installed, it applies the default monospace font on your system.
to a note and it changes the Markdown side, does nothing to the Editor.
I opened userstyle.css and userchrome.css and put the above in. Also tested with and without style tags. I set "Editor font family" to "Courier New" and go to the Editor and it looks same as it ever was, aka the proportional font.
Did you quit Joplin? As in Ctrl-Q? The CSS files only get processed when Joplin first starts fresh. Simply clicking the close button doesn't really close it, you have to quit from the "File" menu (top left) or shortcut Ctrl-Q.
Changing the Markdown Editor font is, as you have seen, done in the settings. The only thing I have found is that because these fields are not drop-down selectors you have to type the name of the installed font exactly as it is shown in your OS font manager. I use "Roboto Mono" and these settings change the text of the Markdown Editor editing pane to the Roboto Mono monospace font.
But people here may be talking at cross-purposes as it is a bit unclear as to what you exactly want to modify. Also you do say,
Tried using Markdown but only get a double view changed
Which suggests that something in "double view" (what do you mean by that please?) did change.
Can I ask:
What OS are you using?
What monospace font do you want to use?
Do you want to change the text to monospace for:
The Markdown Editor editor pane?
The Markdown Editor viewer pane?
Both Markdown Editor panes?
The Rich-Text Editor WYSIWYG pane?
All of them?
Once it's clear what you are trying to do it may be a bit easier to help work out what is going wrong.
Got a font to stay, but using spaces instead of tabs that do not anything and a copy and past that just makes a mess. Giving up on Jopin and looking for something else.
Thanks for the help..