.timeline-panel {
background-color:red!important;
}
.timeline > li > .timeline-panel:after {
border-left:14px solid red!important;
border-right: 0 solid red!important;
}
.timeline > li.timeline-inverted > .timeline-panel:after {
border-left:14px solid #fff!important;
border-right-width: 14px!important;
border-left-width: 0!important;
}
You don't need !important
if you ordered your rules properly and/or modified the values accordingly, but it's there so the rule is shown at the top of the page.