How to apply one class to another?

Question

So I have few elements in .card class and I want to apply w3-card-4 class to it.

.card {
    background-color: #FFFFFF;

}
.w3-card-4 > .card

Answer 1

There should be no spaces in between

.card.w3-card-4{
    background-color:green;
}

.w3-card-4 > .card this is used when element of class="card" is direct child of element with class="w3-card-4"

For the html part. you have to include both classes in html too. like this:

<p class="card w3-card-4"> Text Here </p>

You don't have to include it in html if you are adding it through javascript / jQuery.