Hi,

You can use site CSS stylesheet to make you changes present(http://devnet.kentico.com/docs/devguide/css_overview.htm). I have tried whether styles are applied to list items and I couldn't encounter any issue while using Firefox and IE7.

You can find simple code of styles that I have used for my list bullet images below:

ol{
list-style-type: none;
}

li{
padding-left: 14px;
background-image: url(/31a_test/App_Themes/CorporateSite/Images/print.gif);
background-repeat: no-repeat;
background-position: 0 0px;
}

I have added this code in CMS Site Manager -> Development ->CSS Stylesheets -> edit/create your stylesheet.

However, it seems Firefox is using its own styles when none of default style is specified:

http://www.oppenheim.com.au/2008/07/06/how-to-view-firefoxs-default-internal-css-stylesheet/

Best Regards,
Miroslav Remias