How do I centre a single line of text in a passage?

Completely bamboozled by the Harlowe documentation on this, and I can't find much on the forum either.

I want to centre single lines of text in a number of passages and I cannot for the life of me work it out.

Let's say the line of text is: Put me in the middle of this line

I am confused about whether I need to add some css to the story style sheet (which I've used to change the font and size) or if I'm just supposed to use special characters in the actual body copy (which I've used to bold and italicise).

My knowledge of CSS is absolutely zero so please bare in mind you're explaining something to someone with the understanding of a stone.

I need to centre a number of lines of text that are also bolded and italicised, if that makes any difference.

Thanks in advance.