In HTML5, the email input field is represented as <input type="email" />. The text input field is represented as <input type="text" />. There is a difference in the ‘type’ attribute value.

Review your theme’s CSS style sheets. You will find some items using attribute selector like this:

input[type="text"]
{
	...
}

This ‘input[type=”text”]’ selector doesn’t match the email field because it doesn’t have the ‘text’ type. If you wish to apply this style rule to email fields as well, add a selector that matches email fields:

input[type="text"],
input[type="email"]
{
	...
}

See also: Styling Contact Form