R – How to replace a sifr font of an existing site without changing the font size in the css

Given sIFR 3, it is possible to change some of the CSS used to render the text.

I'm assuming you have two replacements, for h1 and h2, and we're only changing their color:

function changeColor(hexValue) {
  var css = '.sIFR-root { color: ' + hexValue + '; }';
  for (var i = 0; i < sIFR.replacements['h1'].length; i++) {
    sIFR.replacements['h1'][i].changeCSS(css);
  }
  for (var i = 0; i < sIFR.replacements['h2'].length; i++) {
    sIFR.replacements['h2'][i].changeCSS(css);
  }
}

// after switching stylesheet:
changeColor('#FF9900');

This should change the text color of the <h1> and <h2> replaced elements to orange.

The objects returned by sIFR.replacements[][] are FlashInteractors.

sIFR only supports a background color for the entire Flash movie, and this can't be changed later.

Your best bet is to enable transparency for the replacements, and set the background color on the HTML element replaced by sIFR (the <h2>). You're advised however to use transparency only when strictly necessary, and please keep in mind that transparency is largely unsupported on Linux. It will fall back to the specified background-color instead.