Ah, thanks for the explanation -- I thought that you meant that custom.css would be disregarded, now I see you meant that css _rules_ should use "last declaration wins".
Not really a followup on my previous question: what would be done if, say one can't change the order: so custom.css is still before original css. In such cases, without the specificity rule, wouldn't the only option we'd have is to somehow inline the styles, or spray !important everywhere?
To be clear, I only ever did this once for jupyter, and using very specific selectors was the only way I could do it -- of course I learned it hard way why this approach isn't great when jupyter changed the layout slightly. :)
If you're curious, https://github.com/prashnts/dotfiles/blob/0a2e98a848a98b7010... is that custom.css