Center widgets

Hello,
I’m using the “Twenty Twelve” theme and I would really like my side bar’s widgets to be centered when appering on mobile. Currently they appear on the right side (I guess it’s becuase my text is wrriten from right to left), whitch creates an un-balanced look. I tried to change that with CSS but couldn’t find a way.
If anyone knows how to help me I would really aprriciate that!

Best regards