How to use javascript in Selenium

Hi
I am new this forum and would be glad if anyone can help me answer my queries
Thanks!

I am testing a plugin which I added in my webserver which say removes the logo from the webpage (or changes the color of top section of the page) by making the image url empty url() or changing the color property of top section. So I am not sure how I can capture these css changes of these element which are not inline but are specified in the global.css folder.
I know I need to use javascript but not sure how I can add it in selenium code. Or can I add it directly in selenium IDE ?.
Following is javascript working code for FF the alert was just added to check the result in FF)

var style1 = document.defaultView.getComputedStyle(document.getElementById('myLogo'), null).getPropertyValue("background-Image");
alert(style1);


if (style1== "url(images/logo_small.png)")
{document.write="fail";alert("fail");}
else {document.write="pass";alert("pass");}
Any help on this will be much appreciated. I just want to make sure if I can so all this using selenium if not I need to start exploring other tools like watir? DO you think other tools (free) would be better in this case where i need to test css changes on the page?

Thanks!
Hiba