I'm not sure because I don't use the shoutbox but it seem to be editing a CSS file.
If the shoutbox doesn't have class or id CSS tag in there you can specific one into the templates for shoutbox and then you add those tag into your CSS and make adjustment change to it.
one reminder about cp theme, I'm not sure you will have to edit it again if there's another XOOPS updates. You know,... it will overwrite the cp file if there's change.