<style>
  p {
    background-color: blue;
    color: white;
  }
</style>

<style media="all and (max-width: 500px)">
  p {
    background-color: yellow;
    color: blue;
  }
</style>

<p>My background is yellow and my content is blue on browsers up to 500px wide. Otherwise, my background is blue and content is white.</p>

body {
  height: 100vh;
  display: flex;
  place-items: center;
}

p {
  font-size: 1.25;
  line-height: 1.25;
  margin: auto;
  padding: 1rem;
  width: min(90%, 600px);
}