I\'ve been looking at a few different things I\'d like to using JavaScript to tweak styles globally. I\'d like to do this by changing the CSS rule that dictates the element\
To modify your existing styles, either find the stylesheet in document.styleSheets or from the the .sheet property of the or element you want to modify. Then modify the properties in whatever rule they're located in (see https://developer.mozilla.org/en/DOM/CSSRuleList). I'd advice against using the CSSOM to modify properties, as browser support for modifying CSS properties through the CSSOM is pitiful (no browsers whatsoever support it). Instead, just set a string value.
If all you want to do is insert a new rule, just get a stylesheet from the method above, or document.documentElement.appendChild(document.createElementNS("http://www.w3.org/1999/xhtml","style")).sheet. Then use insertRule to add your rule.