You can access the email form through its parent container, the text widget. All text widgets ahve an ID
HTML Code:
div.text-382335242 input {
...
}
Or, you edit the text widget and add a class to the input fields in there
<input name="name"/>
becomes
<input class="text inputblur" name="name"/>
text and inputblur are existing classes. inputblur for a width of 95%