Simple CSS Styling
Add this to your CSS
#mc_embed_signup {
background:#000;
color:fff;
border:1px solid red;
padding:20px;
}
Adjust to Your Preferences
You can adjust the border width by giving it a larger than 1px value, such as this:
border:8px solid red;
These are the colour values used. They are called Hexadecimals (Hex):
#fff
= White
#000
= Black