chrome-extensions-samples/functional-samples/tutorial.getting-started/popup.js

// Initialize button with users' preferred color
const changeColor = document.getElementById('changeColor');

chrome.storage.sync.get('color', ({ color }) => {
  changeColor.style.backgroundColor = color;
});

// When the button is clicked, inject setPageBackgroundColor into current page
changeColor.addEventListener('click', async () => {
  const [tab] = await chrome.tabs.query({ active: true, currentWindow: true });

  chrome.scripting.executeScript({
    target: { tabId: tab.id },
    func: setPageBackgroundColor
  });
});

// The body of this function will be executed as a content script inside the
// current page
function setPageBackgroundColor() {
  chrome.storage.sync.get('color', ({ color }) => {
    document.body.style.backgroundColor = color;
  });
}