You want to alter the style of some text already displayed on the page.

Change one or more of the associated style properties of the element containing the text, as in these examples:

               elementReference.style.color = "00ff00";
elementReference.style.font = 
    "bolder small-caps 16px 'Andale Mono', Arial, sans-serif";
elementReference.style.fontFamily = "'Century Schoolbook', Times, serif";
elementReference.style.fontSize = "22px";
elementReference.style.fontStretch = "narrower";
elementReference.style.fontStyle = "italic";
elementReference.style.fontVariant = "small-caps";
elementReference.style.fontWeight = "bolder";