<div class="block">div.block 1</div>
<div class="block">div.block 2</div>
<div class="inline-block">div.inline-block 1</div>
<div class="inline-block">div.inline-block 2</div>

<button type-"button">
  Add some style
</button>

const style = `
  .inline-block {
    text-decoration: underline;
  }
  .block {
    display: block;
    border: 1px dashed red;
    margin: 2px;
    padding: 2px;
  }
`;

const addStyle = () => {
  const css = document.createElement('style');
  css.innerText = style;
  document.head.appendChild(css);
};

document.querySelector('button')
  .addEventListener('click', addStyle);