if you are lucky, then the widgets have individual css ids or classes to identify them.
you can habe a look into the html code in the browser to find this out; or use a tool such as firebug or the firefox web developer add-on.
for more suggestions, a link to your site showing the issue is needed.
This is my site
Mededelingen has to be green, i did try to add the php, but it changed all the heads and it hasn’t got a individual id nor class…
that ‘mededelingen’ widget has the unique css id:
#wp_category_post_list_itg-4
so you could style the widget title background with a style like:
#wp_category_post_list_itg-4 h3.t{ background:green; }
or with a background image
(add the new style somewhere to style.css of your theme)
