moodle/blocks/messages/styles.css

.block_messages .content {
    text-align: left;
    padding-top: 5px;
}

.block_messages .content .list li.listentry {
    clear: both;
}

.block_messages .content .list li.listentry .user {
    float: left;
    position: relative;
}

.block_messages .content .list li.listentry .message {
    float: right;
}

.block_messages .content .info {
    text-align: center;
}

.block_messages .content .footer {
    clear: both;
}