Hi,
I suggest you to use the Internet Explorer Developer Toolbar for client script and html development, you can get control’s id or name directly.
And what’s more, when using content editor web part to append JS function, please do not forget to add _spBodyOnLoadFunctionNames.push("functionName") function.
See: http://sharepointsolutions.blogspot.com/2007/07/adding-javascript-to-content-editor-web.html
Finally, it is strongly not recommend to modify core.css directly, please achieve your requirement with js or SPD css modification only.
Hope this can help.
Best Regards,
-Aaron