You’re probably looking at an “old” notice.
It looks like this when it’s new
but after a certain amount of time, it becomes old so the color highlight is removed and looks like this
You can control the duration in which post notices are considered new via the
old_post_notice_days site setting which defaults to 14 days.
In other words, after 14 days, unless you change the value for the setting, the notice becomes old and loses the highlight.
You don’t have to worry about that. While it’s still new, a post notice should be highlighted regardless of what theme you use because we use this
which means that new notices will always be highlighted with a derivative of the
tertiary color used in your theme. So they’ll always stand out.