You could also use the !important feature of css to make qualities you do not want to override in the original class. I am using this on my site to keep some of the essential characteristics of the original class while overriding others:
.foo {
color: blue;
width: 200px !important;
}
.bar {
color: red;
width: 400px;
}
This will generate a class "foo bar" element that is red and 200px. This is great if you are using the other two classes individually and just want a piece from each class.