Grrr i hate these new code boxes - i'll just post it in bold. If i picked up on what you were saying then use this:
a.mainmenu {
border: 1px solid #172432;
border-left-width: 0px;
border-right-width: 0px;
color: #C0C9D9;
display: block;
font: 10px verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif;
font-weight: bold;
padding-left: 2px;
width: 100%;
text-decoration: none;
}
a.mainmenu:hover {
background-color: #4D98C1;
border: 1px solid #4DABDE;
border-left-width: 0px;
border-right-width: 0px;
color: #FFFFFF;
text-decoration: underline;
}
Btw you need to add the active, visited one's too (there's one more too which i'm just to lazy to dig out from my memory

)