When viewing old revisions e.g. https://en.wikipedia.org/wiki/Ambondro_mahabo?oldid=914873977 a box appears at the top of the page making sure you are warned that the content you are reviewing is out of date. However this uses bespoke CSS class .mw-revision. We will change these to make use of .warningbox to be consistent with other warning messages we show across the MediaWiki interface so it is clearer to users that there is something of note they should consider while looking at the page.
Note: Since the message inside the content area is customizable by editors via templates, template editors will need to be notified that this change is coming and that they need to prepare accordingly (see the screenshots for how this will look without any input from them):
Ideally editors should remove backgrounds and any text - color. Editors may continue to edit the text as they see fit.