Look out for color contrast
When adding the example CSS to your theme, make sure the colors for links inside the message, the bottom border color, and the various message color schemes don’t introduce color contrast issues into your theme. You’ll want to make sure there’s appropriate contrast between the message foreground and background, as well as apropriate contrast between the message and the surrounding text. You may also want to include a style guide somewhere on your site explaining the significance of each colored message, and add that link to your custom message.
Make sure the importance of the message isn’t communicated by color alone
When adding your custom entry messages, make sure that the importance of the message isn’t communicated by color alone. You can accomplish this by prefacing each message type with a word or series of words indicating its importance: “Important”, or “very important” or “urgent”, for example. You’ll want to make sure you don’t make these indicators visible to just people who use screen readers, since people who live with color blindness or cognitive disabilities also need to know the information you’re alerting everyone else to.
Custom aentry message boxes can be a useful addition to some types of websites. If you’ve never planned for accessibility before, they can be a great place to start developing that habit, since they’re a simple customization. It doesn’t matter if the rest of your site is accessible at this point. These messages are a great place to start making improvements, and to borrow from the philosophy of Breslov Hasidism, one accessibility improvement paves the way to another, and you should make as many accessibility improvements as you can, even if you can’t improve everything at once. Finally,
The work is not for you to finish nor are you free to desist from it.