Calendar Pattern Trouble Shouting

I could use some help with the documentation about the Calendar Pattern. I tried to follow the instruction, but at the part, with the CSS Selectors, I have no clue what they want me to do.

Here is the documentation:

Thank you in advance :)

Hi Kai,

Those are the classes that the pattern uses to change the look and feel of the selected day. If you change those classes you can change how the selected day looks. Lets say you can give it a different color or shape.

For example:

.is-selected button{

    background-color: red;