Custom dropdown menu

Hello All,
Im a novice WordPress Programmer.I have managed to create a custom theme for one of my websites which I earlier designed purely using HTML5,CSS3 and Php.
The website has a dropdown menu purely using CSS styles.
In WordPress I created the custom menu ,but Im hitting a roadblock here as WordPress doesnt seem to recognize the css styles I have used.I gather that the classes used in my styles code for the top level li and the sublevel li is the issue here.
Is there a particular file in wordpress I need to modify to get this dropdown working
following is my css code

/*dropdown menu styles */
#mainNav ul.subMenu {
float:none;
background:#699;
width:auto;
height:auto;
position:absolute;
top:2.3em;
left:-9000em;
max-height:0;
-moz-transition:max-height .5s ease-in-out;
-webkit-transition:max-height .5s ease-in-out;
-o-transition:max-height .5s ease-in-out;
transition:max-height .5s ease-in-out;
overflow:hidden;
}
#mainNav ul.subMenu li {
float:none;
}
#mainNav .topMenu li:hover ul {
left:0;
max-height:10em;
}
#mainNav ul.subMenu li a {
color:white;
font-size:.8em;
border-bottom:1px solid #7DAAA9;
padding:.2em 1em;
white-space:nowrap;
}
#mainNav ul.subMenu li:last-child a {
border-bottom:none;
}
#mainNav ul.subMenu li a:hover {
background:#7DAAA9;
height:2.3em;
padding-top:.2em;
top:0;
border-radius:0;
}