It's caused by this:
.ui-widget-content {
border: 1px solid #AAA/*{borderColorContent}*/;
background: #FFF/*{bgColorContent} url(images/ui-bg_flat_75_ffffff_40x100.png)/*{bgImgUrlContent}*/ 50%/*{bgContentXPos}*/ 50%/*{bgContentYPos}*/ repeat-x/*{bgContentRepeat}*/;
}
It's in the jquery-ui.css
file around line 246.
An alternative is to change your own CSS. Try changing this:
#accordion .ui-accordion-content {
background-color: #777;
}
to this:
#accordion .ui-accordion-content {
background: #777;
}