The classes are as follows:
.contactform
This is the overall contact form. If you want a background behind the form, you would put it here.
.contactleft
This is where the labels for the input boxes (Name, etc.) are.
.contactright
This is where the input boxes are located.
.contactform input
This would be the class for the input boxes (everything except for the message box).
.contactform textarea
This is the class for the message box.
For an example, this is what I have for my css for this plugin:
.contactform {
position: static;
overflow: hidden;
background: url(images/cformbg3.jpg) no-repeat;
padding-top: 21px;
height: 442px;
}
.contactleft {
width: 20%;
text-align: right;
clear: both;
float: left;
display: inline;
padding: 4px;
font: small-caps bold 13px Georgia, "Times New Roman", Times, serif;
color: #fff;
}
.contactright {
width: 70%;
text-align: left;
float: right;
display: inline;
padding: 4px;
}
.contacterror {
border: 2px solid #ff0000;
}
.contactform input {
border: none;
background: transparent url(images/cformbg.gif) no-repeat;
padding: 3px 8px;
height: 22px;
width: 230px;
font-family: arial, helvetica;
font-size: 12px;
}
.contactform textarea {
margin:3px 5px;
padding: 5px;
background: transparent url(images/cformbg2.gif) no-repeat;
border: none;
font-family: arial, helvetica;
font-size: 12px;
width: 280px;
height: 200px;
}
Make sure you disable the css option on the plugins options page if you want to use your own css.