Support package specific themes

[gaborcsardi]

So a package could register a theme, and then this would be used when printing errors from that package, using topenv().

This is still not ideal, though. E.g. if I define custom formatting for a (say) processx::process object, then that should be used, always, when formatting such an object.

Maybe cli_format() or rather a similar function could return a style instead of the formatted string, and then this style would be applied for .val? And the function would be S3, so it would be always used.

So these are actually two different features.

[sda030]

If I can chip in on your first statement, if I understand you correctly. I think it would lead to a nightmare if each package can implement their own formatting theme for their error and warnings. Not only would it be difficult for beginners and users with visual impairments to struggle with various ways of communicating the same thing, but also for regular users who prefer consistency. But beyond errors and warnings, I would not have nightmares. :) (P.S. Among my top-5 favourite packages)