Thanks, that's interesting to know.
I don't see how I can turn that into a solution for the core problem though, which remains that pluma is not using the menu font for the menu.
To be clear: this is not about the menu being monospace, it's about it using the wrong font entirely, proportional or not. There is a font specified for menus. pluma (via GtkSourceView) is not using it for that context menu, and it should be.
Since the menu font is system-wide and not part of the pluma theme, no amount of changing the css will fix it unless there's a solution along the lines of "font-family: use-the-menu-font" - what I'm looking for (though not expecting to actually find) is what that syntax is.
Hopefully that's clearer?