Twine uses HTML so styling is done with CSS.
To change the style of the whole game, create a passage and tag it "stylesheet"; put the CSS in there (the best way to do that is to Google stylesheets other people have created for your Twine format (e.g., Sugarcane), and paste that in, and then to modify it to suit your story.
If you have a set of passages that need an alternative style, create a second passage tagged as "stylesheet", but give it another tag too. Any pages with that new tag will automatically be styled according to the second stylesheet, rather than the first.
If you have sections with passages that need an alternative style, the best way seems to be to wrap them in a <div> and <span>, with a class defined in your stylesheet.
Here is a link to some example stylesheets: