Hello Maria,
This is automatically set in the index.php file and pulled from your highlight color entered in the Vertex admin. If you want to change it to a different color then you can edit the following lines and remove the php that is outputting your highlight color:
This is for the border above the active menu item:
#s5_menu_inner #s5_nav li.active a {border-color:#<?php echo change_Color($s5_highlightcolor,'+25'); ?>;}
The hover color for each item is set in the index.php file too but its in a long CSS list of items set so you'll just have to look for "#s5_nav li:hover a" in the index.php file and remove this. You'll see its in the middle of a big list of items that have the highlight color applied to them.