These days, many themes offer a simple solution to add your email form code to the themes custom opt in boxes. They may also include a special widget which has been created especially for that theme.
All you need to do is copy your form code from Aweber or MailChimp etc and paste it into the box.
What about if you want to use your feed like feedburner rather than email service? You could paste the default code in directly from Feedburner but it may not look to good.
Here's some code which has been modified.
The subscribe box below this post uses this code which you could easily modify for your own use if you're not using Thesis.