TwenfyFifteen child theme does not change the font

Following the Theme Development with Inheritance documentation, I created a child theme for TwentyFifteen.

I would like to change the font for body text from "Noto Serif" to "Noto Sans", however no change is taking place.

Am I missing anything obvious, or are there common errors that I may have made?

/css/custom.css file:

 /**
 * 3.0 Typography
 */

 body,
 button,
 input,
 select,
 textarea {
     color: #333;
     font-family: "Noto Sans", sans-serif;
     font-size: 15px;
     font-size: 1.5rem;
     line-height: 1.6;
 }

h1,
h2,
h3,
h4,
h5,
h6 {
    clear: both;
    font-family: "Noto Serif", serif;
    font-weight: 700;
}

@pamtbaau,

Thank you. This worked. I am not a web dev, so for the sake of others in a similar situation:

1. I copied twentyfifteen/templates/partials/base.html.twig to /twentyfifteen-child/templates/partials/.
2. Added the line posted by @pamtbaau to the {% block stylesheets %} block of the newly copied base.html.twig file.