[satch]

You missed another equal sign to create the comparator == on the conditional, resulting to the visibility being always hidden for the themeToggleLight object. Furthermore, you need to make the themeToggleDark visible if you’re testing for themeToggleLight to be hidden.

Here’s the fixed code:

function ToggleTheme() {
    var themeToggleDark = document.getElementsByClassName("theme-toggle-dark")[0];
    var themeToggleLight = document.getElementsByClassName("theme-toggle-light")[0];

    if (themeToggleLight.style.visibility == "hidden") {
        themeToggleDark.style.visibility = "hidden";
        themeToggleLight.style.visibility = "visible";
    }

    else {
        themeToggleDark.style.visibility = "visible";
        themeToggleLight.style.visibility = "hidden";
    }
}

<div style="max-width:50px;">
  <svg class="theme-toggle-dark" alt="Icon used for changing site theme to dark" onclick="ToggleTheme()" aria-hidden="true" focusable="false" data-prefix="far" data-icon="moon" class="svg-inline--fa fa-moon fa-w-16" role="img" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path fill="currentColor" d="M279.135 512c78.756 0 150.982-35.804 198.844-94.775 28.27-34.831-2.558-85.722-46.249-77.401-82.348 15.683-158.272-47.268-158.272-130.792 0-48.424 26.06-92.292 67.434-115.836 38.745-22.05 28.999-80.788-15.022-88.919A257.936 257.936 0 0 0 279.135 0c-141.36 0-256 114.575-256 256 0 141.36 114.576 256 256 256zm0-464c12.985 0 25.689 1.201 38.016 3.478-54.76 31.163-91.693 90.042-91.693 157.554 0 113.848 103.641 199.2 215.252 177.944C402.574 433.964 344.366 464 279.135 464c-114.875 0-208-93.125-208-208s93.125-208 208-208z"></path></svg>
  <svg class="theme-toggle-light" alt="Icon used for changing site theme to light" onclick="ToggleTheme()" aria-hidden="true" focusable="false" data-prefix="far" data-icon="sun" class="svg-inline--fa fa-sun fa-w-16" role="img" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path fill="currentColor" d="M494.2 221.9l-59.8-40.5 13.7-71c2.6-13.2-1.6-26.8-11.1-36.4-9.6-9.5-23.2-13.7-36.2-11.1l-70.9 13.7-40.4-59.9c-15.1-22.3-51.9-22.3-67 0l-40.4 59.9-70.8-13.7C98 60.4 84.5 64.5 75 74.1c-9.5 9.6-13.7 23.1-11.1 36.3l13.7 71-59.8 40.5C6.6 229.5 0 242 0 255.5s6.7 26 17.8 33.5l59.8 40.5-13.7 71c-2.6 13.2 1.6 26.8 11.1 36.3 9.5 9.5 22.9 13.7 36.3 11.1l70.8-13.7 40.4 59.9C230 505.3 242.6 512 256 512s26-6.7 33.5-17.8l40.4-59.9 70.9 13.7c13.4 2.7 26.8-1.6 36.3-11.1 9.5-9.5 13.6-23.1 11.1-36.3l-13.7-71 59.8-40.5c11.1-7.5 17.8-20.1 17.8-33.5-.1-13.6-6.7-26.1-17.9-33.7zm-112.9 85.6l17.6 91.2-91-17.6L256 458l-51.9-77-90.9 17.6 17.6-91.2-76.8-52 76.8-52-17.6-91.2 91 17.6L256 53l51.9 76.9 91-17.6-17.6 91.1 76.8 52-76.8 52.1zM256 152c-57.3 0-104 46.7-104 104s46.7 104 104 104 104-46.7 104-104-46.7-104-104-104zm0 160c-30.9 0-56-25.1-56-56s25.1-56 56-56 56 25.1 56 56-25.1 56-56 56z"></path></svg>
</div>

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