CSS Help on Jetpack Twitter Widget

I’ve added the Jetpack Twitter widget but the font is too small and the wrong color. I’ve tried to edit this by checking what the relevant class on the CSS code would be via Firebug and found this:
.widget_twitter li {
list-style-type: none;
margin-bottom: 14px;
}

I tried to add this in to my child-theme style sheet:

.widget_twitter li
{color:#ffffff;
font-size:18px;
}

but it didn’t work. Could someone please help me out with this?
My site is swasthyamundial.com

Thanks,
Abraar