You can prepend your suffix to a css rule in order to override it. For example, if the original css rule is:
.mwall-big .mwall-item-inner h3.mwall-title a {
font-size: 20px;
}
then you can override it with
.myclass .mwall-big .mwall-item-inner h3.mwall-title a {
font-size: 22px;
}