A bit late in replying, I don’t know if you are still here, I have not great magic to teach you unfortunately. The only way I know for integrating a CSS file in Plone directly TTW is to add it to the theme CSS file. It’s a bit difficult with the default Plone 5 theme (Barceloneta) so I use a different theme that I installed in a classic way (buildout). Then if I want to fiddle with the styling I just add a few instructions at the end of the (compiled) file. It’s monstrous but I do it. You could rename the css file for your theme and create a new file with the same name and import the renamed file if you want to do it in a (slightly) cleaner way.
I have also pointed at another method in a different thread: the ambidexterity way that could be used to setup something like your example.
That’s all I know about ‘non technical’ ways of fiddling with CSS, if you can call using CSS ‘non technical’ (IMO it’s harder than Python but your taste may be different of course)