Toggling between light and dark mode using JavaScript
One thing that you could update is how you are using the ternary operator for isDark.
Ternary operators are used for returning something based on a boolean response, so either return X if true, or Y if false.
However, every time it is used it is evaluating the statement being passed to it. In this case both times it is evaluating isDark. For the average website, this won't affect your processing time, but it is something to think about.
Instead a classic if/else statement would allow you to run more code off a single evaluation.
You will notice this isn't as sleek looking as ternary and is even more lines of code, but gives more capabilities when you need it.
if (isDark) {
  toggleIcon.children[1].classList.replace('fa-sun', 'fa-moon')
  imageMode(darkLightTheme[0])
} else {
  toggleIcon.children[1].classList.replace('fa-moon', 'fa-sun')
  imageMode(darkLightTheme[1])
}