You can overwrite this snippet by placing a custom version in site/snippets/email-manager/fields/date-range.php.
Modify the CSS classes or structure to match your project’s requirements
<div class="<?= $field->className('daterange-wrapper') ?>">
<input type="date" <?= $field->attr('start') ?> name="<?= $field->name() ?>[start]" value="<?= $field->value('start') ?>" />
<input type="date" <?= $field->attr('end') ?> name="<?= $field->name() ?>[end]" value="<?= $field->value('end') ?>" />
</div>