I was able to get as far as this by using the CSS code below, copied from rounded text_input and by adding #cccccc to border-style.
#mc_signup_form #mc_mv_EMAIL, #mc_mv_FNAME, #mc_mv_LNAME {
border-width: 1px;
border-style: solid #cccccc;
font-family: Arial, Helevtica, Verdana, san-serif;
font-size: 12px;
padding: 8px 12px;
-moz-border-radius: 5px;
-webkit-border-radius: 5px;
border-radius: 5px;
}
But as you'll see, it's still not exactly the same--the top and left sides of the textbox are of a different color.
How I want it to look like (Search):

How it looks like now (MailChimp):

Am I missing anything? I'm not really knowledgeable on CSS and am just guessing how to use it based on my knowledge of HTML and other programming languages.
Thanks!