Note that the right margin is always bigger then the left one, because it also adds a space for the eventual scrollbar, even when it is not needed, which is annoying too (especially in small cards like the yellow one in image) because you can’t have some text symetrically centered inside of a card.
I tried this snippet:
.canvas-node-content { margin-left: -12px; }
But it presents two problems: first, it moves also the background, and second, “margin-right” stangely enough doesn’t seem to do anything.
Yeah, rendered canvas cards are a bit tricky because of the content-blocker layer. You can dig down under that in the Elements tree or, a neat trick, with the canvas-node-content-blocker layer selected, click delete on your keyboard to temporarily remove the layer and get to the elements below it.