The preview of the broadcast message, with the 2px thick black lines no
longer matched the style of the area labels, which have moved to just
having a tinted background.
This commit restyles the broadcast message preview (including the
exclamation mark icon) to use shading only to differentiate the message
from the rest of the page.
This should make the page fit together more cohesively, and feel less
busy.
This is borrowed from an earlier prototype, and is meant to be temporary
– something better than just plain text.
Text in generated content isn’t always announced by screen readers, so
we should definitely move away from that once we understand what text
will be shown on the phone and where it comes from.
