This conversation on 30/31 August 2021 has some useful info about Custom CSS:

manton: I’ve fixed the “squished photos” problem in the new Ink theme. It’s now in a plug-in so it’s easier to install. Click Plug-ins → Find Plug-in. Also, clear your default theme by setting it to “Blank” under Design.

odd: Will the custom CSS be removed if I do that?

manton: Nope, if you’re using the “Edit CSS” button that lives separately from any themes and will always be applied.

manton: “Edit CSS” basically creates a new CSS file and applies it on top of whatever theme you’re already using, so it’s a good place to override changes without messing with anything else.