Increase timeout on theme activation snackbar

Quick summary

When you activate a theme a message briefly appears to tell you it's successful and to give you a chance to go and look at your site with its shiny new theme.

This message disappears after ten seconds and you can't bring it back. It's arguably an accessibility failure (WCAG 2.2 Guideline 2.1: Enough time) though there are other ways to view your site, and to realise what the active theme is.

The timeout is configurable and we did note when implementing it that we should review this based upon feedback.

Design's initial guidance (p1725899556548679/1725851524.314329-slack-CRWCHQGUB) was:

snackbars are best for brief and non-critical notifications, like in this case. If they don’t contain a next step, they should auto-close after a few seconds. Otherwise we should keep them visible so users can click if they want

It's a grey area here because there's an optional next step. I think we should probably remove the timeout, but at the least, increase it to 20 seconds.

This was initially prompted by some feedback from @lizswafford : paYJgx-5CF-p2#comment-5604

Steps to reproduce

Go to wordpress.com/themes
Choose a theme
If present, click "Select a site for this theme" and choose a site
Click "Activate this design"
Click "Activate [theme name]" on the modal
Notice the snackbar appear in the top right

What you expected to happen

The snackbar to stay around until I dismissed it

What actually happened

The snackbar disappeared, to view what my site looks like in the theme I selected there's at least two clicks. Most quickly by hovering over the site's name in the masterbar and then clicking "view site"

Impact

Some (< 50%)

Available workarounds?

Yes, easy to implement

If the above answer is "Yes...", outline the workaround.

Assuming you noticed and read the text in ten seconds, you can view the site in many ways.

Platform (Simple and/or Atomic)

No response

Logs or notes

No response

Automattic / wp-calypso