How to tweak EWW readable mode?

[Pengji Zhang <kunhtkun@gmail.com>]

Hi,

I am using Emacs master, and I really like the changes to EWW readable
in 72972118e6f5831f200108cd7b80bf86538c265e and
4b0f5cdb01fbd05c8184a89fa8543eb5600fb4f8. Many thanks to Jim!

However, now I could not find an easy way to tweak readable mode. For
example, I would like to use a narrower width for a web page only in
readable mode. I used to add an `:around' advice to `eww-readable':

    (define-advice eww-readable (:around (fn &rest args))
      (let ((shr-max-width 60))
        (apply fn args)))

Now `eww-readable' could toggle readable mode and pages may also be in
readable mode by default. So that advice does not work any more.

I wonder if there is a reliable way to tweak the rendering and apply
some settings only in EWW readable mode?

Thanks!

Pengji