Improve styling for digest page

[Shoumorup]

This is with respect to issue #11016 (#11016)

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[timabbott]

@Shoumorup so, I'm worried you've misunderstood the issue; this looks like this substanstially changes the styling of the digest emails themselves. We don't want to do that directly before we apply the inline-email-css infrastructure to that email (which I think @hackerkid has a PR that needs finishing for). #11016 was about applying externals CSS to the /digest page (which is supposed to display the email) to put it inside a centered container to look nicer.

[hackerkid]

I have left a comment in #11016 which has some technical details on how this should be done.

[zulipbot]

Heads up @Shoumorup, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.