Posting this to hopefully help someone else, as I didn’t see anyone else explaining the issue.
I wanted to add a simple CSS snippet to left justify images in my notes. They are centered by default; at least on my set up it was the default though I have a theme installed.
But when I added a CSS snippet file to my Vault CSS Snippet folder and loaded the snippet in settings, nothing changed.
I have the NORD theme installed, and noticed that the theme CSS file had an “img” section for changing certain aspects of CSS for images. I noticed it had a margin-left section, so I changed the margin to “0pt” instead of “auto” and voila, my images are now left justified.
So the lesson is that you may need to edit your Theme folder CSS file as the CSS Snippet feature does not seem to override the Theme CSS. Maybe there is a way to override that somehow, but it was very easy to just edit the Theme CSS instead.