Hi,
as for your menu & colors I went to your custom.css and I found your navigation edits, I changed it like this:
/*navigation*/
/* ACHTERGROND MENU-BALK (HEADER MET DROP-DOWN-MENU-BUTTONS) */
/* --------------------------------------------------------- */
#top .main_menu .avia_mega{
position:relative;
background:#c0c0c0;
z-index:100;
float:left;
height:10px;
border:1px solid #ff0099;
padding:0 !important;
}
/* ACHTERGROND MENU-BALK (HEADER MET DROP-DOWN-MENU-BUTTONS) */
/* --------------------------------------------------------- */
.main_menu .avia_mega, .main_menu .avia_mega ul{
margin:0;
margin-top:20px !important;
padding:0;
list-style-type:none;
list-style-position:outside;
position:relative;
line-height:34px;
z-index:5;
}
That should account for the border & the background, as for the slider you should try and add a full width slider or just a slider to your page through either your template builder or your predefined templates, I can provide specific instructions on styling this once you've got this in action.