Accessibility: Themes: Consistently use an underline for "Enable auto-updates"...

Accessibility: Themes: Consistently use an underline for "Enable auto-updates" link and theme author link in theme overlay.

Props chetan200891, sabernhardt, afercia.
Fixes #50697.
Built from https://develop.svn.wordpress.org/trunk@48564


git-svn-id: https://core.svn.wordpress.org/trunk@48326 1a063a9b-81f0-0310-95a4-ce76da25c4cd