You can overwrite this snippet by placing a custom version in site/snippets/email-manager/fields/select.php.
Modify the CSS classes or structure to match your project’s requirements.
<select <?= $field->attr() ?>>
<?php if (!$field->required): ?>
<option class="placeholder" value=""><?= $field->placeholder ?></option>
<?php endif; ?>
<?php foreach ($options as $key => $label): ?>
<option value="<?= $key ?>" <?= $field->value() === $key ? 'selected' : '' ?>><?= $label ?></option>
<?php endforeach; ?>
</select>