You would need to place the !important after the property you want to set important. 

Example:

.ow_main_menu li.active a, .ow_main_menu li a:hover { 

  color: #ffffff; 

  /** OW_Control type:color, key:menuHoverColor, section:4. Menu, label:Menu hover text color **/ -         

  webkit-transition: background-color linear .1s; 

  background-color: #a9d300 !important; /* if you want this to be important */

  text-shadow: 1px 1px 0 rgba(0, 0, 0, 0.2); 

}

Let me know if you need more help!