1) Number 2 will resolve this, but for full control please test this CSS:
input#mc-embedded-subscribe {
color: #fff;
}
input#mc-embedded-subscribe:hover {
color: #fff;
}
2) Theme Options => Newsletter/Subscribe Bar Options please remove this [mc4wp_form id="15223"] from Newsletter/Subscribe Bar Form Code. You need to use the raw code from Mailchimp. It should be this, you can replace the action url with your own:
<form action="//swiftideas.us2.list-manage.com/subscribe/post?u=a3950048c69f7be72a27013a9&id=68a002176a" method="post" id="mc-embedded-subscribe-form" name="mc-embedded-subscribe-form" class="validate" target="_blank" novalidate="">
<input type="email" value="" name="EMAIL" class="required email" id="mce-EMAIL" autocomplete="off" placeholder="Enter your email address" title="Enter your email address">
<div id="mce-responses" style="display: none;">
<div class="response" id="mce-error-response" style="display:none"></div>
<div class="response" id="mce-success-response" style="display:none"></div>
</div>
<div style="position: absolute; left: -5000px;"><input type="text" name="b_a3950048c69f7be72a27013a9_68a002176a" tabindex="-1" value=""></div>
<input type="submit" value="SUBSCRIBE" name="subscribe" id="mc-embedded-subscribe">
</form>
3) The above will resolve that.
.