No background on Partner/Logo element

Hi,
just added a partner/logo element with a slideshow of some logos. After I’d added a link for each logos, a background appeared behind every image.
I found this style in enfold.css:

.main_color .avia-content-slider .slide-image {background:#fafafa;}

I tried to turn it into #ffffff or #ffffff!important in my custom.css but it didn’t work.

What else can I do?