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Can you give more details? I can see how to enable custom CSS, but where I go from there? Where to put the custom CSS?

Settings -> Appearance. There will be a section to choose a folder with your custom css files.

I may be obtuse, but I am not seeing that on Vivaldi 4.0 on the Mac?

https://i.imgur.com/21ueqSf.png in the lower part of the window (I'm too lazy to switch the language sorry).

You need to restart it for the checkbox to take effect.