addClass function / multiple "div" . please help

Hello friends (ºº)/

How the addClass function is used when multiple "div" and you only want to modify one? ... For example "background-moon"
<div class="background-sky"></div>
<div class="background-star"></div>
<div class="background-sun"></div>
<div class="background-star03"></div>
<div class="background-moon"></div> <== this
<div class="background-site"></div>
Thanks for the help