This is our code, make sure you have the same elements and classes, you shouldn’t need to define any additional CSS:
<div class="clearfix" id="header-subscribe">
<!-- Begin MailChimp Signup Form -->
<div id="mc_embed_signup">
<form novalidate="" target="_blank" class="validate" name="mc-embedded-subscribe-form" id="mc-embedded-subscribe-form" method="post" action="http://...">
<label for="mce-EMAIL">Subscribe to our mailing list</label>
<input type="email" required="" placeholder="email address" id="mce-EMAIL" class="email" name="EMAIL" value="">
<div class="clear"><input type="submit" class="sf-button accent slightlyrounded" id="mc-embedded-subscribe" name="subscribe" value="Subscribe"></div>
</form>
</div>
<!--End mc_embed_signup--> </div>
Regards,
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Cosmin – Support