You can use the period (dot) to refer to a specific class. If you wanted to apply the same styles to multiple selectors, you can seperate them with a comma
In this instance:
/*Target any li element with a class of header*/
li.header {
}
/*Target any li element with a class of menu-item*/
li.menu-item{
}
/*Target any li element with a class of menu-item or header*/
li.menu-item, li.header{
}