How to change the bullet ?

Hi,

I created a Twenty Eleven child theme and I want to change the bullet in the widgets.
I added this line in the style.css file of the child theme but the result is not correct:
.widget li {
background: url(images/red_dot.gif) no-repeat 0 0; */
}
I always see the grey square with the new red dot bullet above the texte.
How can I replace the grey square bullet by a new bullet ?

Thanks for your help.