Hello,
Please read this article – https://xstore.helpscoutdocs.com/article/91-mail-chimp-form-custom-styles here is an example:
https://gyazo.com/193c20ab6ae06baf50fcccb0557db9d8
<div class="et-mailchimp classic-button ecotransport">
<input type="email" name="EMAIL" placeholder="Enter email" required />
<input type="submit" value="Subscribe now" />
</div>
Regards