[localhost2020]

Привет!
как добавить document.getElementByClassName к следущему коду: Учитывать нужно чтобы было и documen.getElementById и document.getElementByClassName Спасибо!

<div class="themes">
    <button class="gray" data-theme="gray">GRAY</button>
    <button class="red" data-theme="red">RED</button>
    <button class="green" data-theme="green">GREEN</button>
    <button class="blue" data-theme="blue">BLUE</button>
    <button class="unknown" data-theme="unknown">unknown</button>
</div>

<div id="opops">Testing</div>
<div id="opopops">Testing</div>

const themes = {
    gray: 'theme--gray',
    red: 'theme--red',
    green: 'theme--green',
    blue: 'theme--blue'
}

const themeElements = [
    document.getElementById('opops'),
    document.getElementById('opopops'),
];

const themeFromStorage = localStorage.getItem('theme');

const defaultTheme = themes[themeFromStorage] || themes.gray;

themeElements.forEach(element => element.classList.add(defaultTheme));

const themeSwitchers = document.querySelectorAll('[data-theme]');

for (const switcher of themeSwitchers)
    switcher.addEventListener('click', switchThemeHandle);

function switchThemeHandle(e) {
    const { theme } = e.target.dataset;
    
    if (!Object.keys(themes).includes(theme)) {
        console.error('Ошибка смены темы. Тема не найдена.');
        return;
    }

    themeElements.forEach(element => {
        element.className = element.className.replace(/([theme\-\-]+[a-z]+)/, '');
        element.classList.add(themes[theme]);
    });
    
    localStorage.setItem('theme', theme);
  
    
}

.themes {
    position: fixed;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
    display: flex;
}

.themes > * {
    margin: 0 10px;
}

.themes button {
    padding: 12px 24px;
    border-width: 1px;
    border-style: solid;
    outline: none;
    color: #fff;
    cursor: pointer;
}

.themes .gray { background-color: #95a5a6; border-color: #7f8c8d; }
.themes .red { background-color: #e74c3c; border-color: #c0392b; }
.themes .green { background-color: #2ecc71; border-color: #27ae60; }
.themes .blue { background-color: #3498db; border-color: #2980b9; }
.themes .unknown { background-color: #9b59b6; border-color: #8e44ad; }

#opops.theme--gray, #opopops.theme--gray { background-color: #95a5a6; }
#opops.theme--red, #opopops.theme--red { background-color: #e74c3c; }
#opops.theme--green, #opopops.theme--green { background-color: #2ecc71; }
#opops.theme--blue, #opopops.theme--blue { background-color: #3498db; }


#opops, #opopops {
    width: 200px;
    height: 50px;
    background: red;
    position: absolute;
    top: 0;
}

#opops {
    left: 0;
}

#opopops {
    right: 0;
}

как сделать чтобы работал с ByID и ByClassName