Quick-apply an existing style to current page, prefix, or domain

[andrew-vant]

I am not sure if this is a feature request, or a failure on my part to find a feature that already exists. I have a tiny snippet of CSS that I reach for very frequently. It restricts body width, for sites that have overly long lines:

body { max-width: 80ch; margin: 5% auto; }

Obviously I can't make this 'style' global; it would break lots of pages. But going through "write style for {whatever}" manually each time I run into an offending site is annoying. What I'd like is a way to quickly add the current page/urlprefix/domain to an existing style. An option just above or below "write style for" in the UI would be ideal. A keyboard shortcut might be even better.

Does this exist? If not, is it something that could be added?