I'm working on a complex GUI where new users are common and so there is a line that either gives instructions, prompts you with information after you select a button that causes a reaction, or prompts you about errors. Prior to me (a UI Designer) the prompt line was at the very bottom of the screen and our users commonly didn't know it existed. On my recommendation we moved it to the top of the page, just below the page title. Due to the occasionally short length of the prompt I chose to center the text so that it would flow from page title to prompt line to page content. I am however getting pushback that the prompt line should be left aligned. My concern for that is if it is a short prompt, it will be to the far left and not be noticed. Could I get your thoughts?
The message should be centered scoping to the entire content width below. Around the message you can have the yellow background highlight as illustrated by Monomeeth and as how gmail does. While the width of the text may vary (short or long), the width of the highlighter may be longer (can be fixed with respect to the browser width) at most times. The purpose of the highlight is to grab attention and it helps no matter how long your message is. Isolate the purposes here and your problem is solved independently. Hope this is what you're looking for.
It can be very frustrating when people (managers, users, etc) don't grasp the intent behind something. I've often found though that there's a way to keep most people happy by finding a solution that achieves the intended goal while also meeting the idiosyncratic views we're all guilty of at times.
I would just use a highlighted header of some sort. Below are three examples of how this could look:
Obviously the colours, icons or wording would depend on your needs. But this approach does achieve your main aim of ensuring that users don't miss the prompt. It's also less likely to upset stakeholders as this is an approach they're probably familiar with from other software they use.
I also think it works better than the original approach of centering your short prompts (my opinion, of course), and that it provides you (the UI designer) more flexibility because you could introduce a level of highlight colours depending on the need. For example, my images above are all a pale yellow, but you could also have pale blue banners to signify a different kind of prompt, a pale red banner to indicate something critical, etc.
Hope this helps. Good luck.