Hi boufon,
The background color is defined in the stylesheet layout.css. So in order to change you would add the following code to your custom.css or Quick CSS. I'm going to list all the social media icons here in case you want to change any other colors.
.social_bookmarks .rss:hover {
background-color: #FFA133;
}
.social_bookmarks .facebook:hover {
background-color: #37589B;
}
.social_bookmarks .twitter:hover {
background-color: #46D4FE;
}
.social_bookmarks .mail:hover {
background-color: #9FAE37;
}
.social_bookmarks .dribbble:hover {
background-color: #E44885;
}
.social_bookmarks .linkedin:hover {
background-color: #419CCA;
}
.social_bookmarks .search:hover {
background-color: #222222;
}
.social_bookmarks .gplus:hover {
background-color: #DE5A49;
}
#top .social_bookmarks li:hover a {
background: url("../images/layout/icon-social-reflect.png") no-repeat scroll 0 top transparent;
}
Regards,
Mya