<form>
    <div class="mb-3">
        <label for="name">[[admin/appearance/skins:custom-skin-name]]</label>
        <input type="text" name="custom-skin-name" class="form-control">
    </div>

    <div class="mb-3">
        <label for="_variables">[[admin/appearance/skins:custom-skin-variables]]</label>
        <textarea name="_variables" class="form-control" rows="20"></textarea>
    </div>
</form>