increase footer widget size

I’m trying to display an amazon iframe in the footer area of enfold, but the widget area is to small (to much margin left). I looked at this issue https://kriesi.at/support/topic/adjust-footer-widget-width/ but unfortunately it didn’t work in my case).

Now I tried to change following css via my child’s style.css (or quick css) but it is simply ignored:

div .av_one_fourth {
margin-left: 0% !important;
width: 25% !important;
}

changing this css with firefox inspector does exactly what I want to accomplish. Any suggestions how to implement the css code correctly in my child’s theme?

Thanks a lot,
and yes – browser cache has been cleared out completely.