CSS coding on Removing Drop Down menu

I am needing to remove the drop down menu from my website while keeping it so that it highlights which tab the customer is on. The only way I have found to do this is CSS and I have never edited the CSS before. I have found the “menu” section and even where it says “drop down.” What do I do from here? I am using the simple catch theme. Below is the code on the menu. Thank you.

/* =Menu
———————————————– */
#header #mainmenu {
float: left;
border:1px solid #ccc;
background-color:#fff;
margin-top:40px;
position:relative;
width:976px;
z-index:1000;
box-shadow:0 1px 0px #fff;
}
.home #header #mainmenu {
box-shadow: none;
}
#header #mainmenu ul {
margin: 0px;
list-style: none;
}
#header #mainmenu ul li {
border-bottom: 1px solid #CCCCCC;
border-right: 1px solid #CCCCCC;
float: left;
margin-bottom: -1px;
position: relative;
}
#header #mainmenu ul li a {
padding: 0 15px;
display: block;
color: #fff;
font-size: 15px;
line-height: 48px;
color: #444;
float: left;
white-space: nowrap;
}
#header #mainmenu ul li a:hover,
#header #mainmenu ul li.current-menu-item a,
#header #mainmenu ul li.current-menu-parent a,
#header #mainmenu ul li.current_page_item a,
#header #mainmenu ul li.current_page_ancestor a,
#header #mainmenu ul li:hover > a {
background-color: #444;
color: #fff;
}

/* Dropdown menu */
#header #mainmenu ul li ul, #header #mainmenu ul li:hover ul li ul {
display:none;
list-style:none none;
z-index: 9999;
}
#header #mainmenu ul li:hover ul, #header #mainmenu ul li ul li:hover ul {
display:block;
}
#header #mainmenu ul li ul {
position:absolute;
background-color:#444;
top:48px;
left:0px;
padding:0px;
width:185px;
margin:0px;
border-right: 1px solid #555555;
border-left: 1px solid #555555;
border-top: 1px solid #555555;
}
#header #mainmenu ul li ul li ul {
left:185px;
top:-1px;
}
#header #mainmenu ul li ul li {
float:none;
border-bottom:1px solid #666;
border-right:0 none;
}
#header #mainmenu ul li ul li a {
padding: 7px 10px;
color: #999;
float:none;
line-height: 18px;
font-size: 13px;
font-weight: normal;
white-space: normal;
}
#header #mainmenu ul li ul li a:hover, #header #mainmenu ul li ul li:hover > a {
background-color:#333;
text-decoration:none;
color:#fff;
}