Put your css on some place whre it will not be overridden by other rule. Good place is bottom of template_css.css or extreme - put css block in head section of page - <style type="text/css" > ...</style> block just before </head> in index.php of template. (just copy css you want to that place - no mater if it be on to places , last one read wins)
Also you can add !important; flag to css property:
.mycssclass{
background-color:#aaaaaa!important;
}