<div>
  <h1>Clique no botão abaixo e altere o seu estilo:</h1>
  <button id="novaCor" onClick="cliqueBtn()">Clique aqui</button>
</div>

button {
  font-size: 18px;
  font-weight: 600;
  background-color: aqua;
  border-radius: 20px;
  border: 0;
  padding: 10px 60px;
  cursor: pointer;
}
function cliqueBtn() {
  document.getElementById("novaCor").style.background = "red";
  document.getElementById("novaCor").style.color = "white"
}

External CSS

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External JavaScript

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