I finally made the recaptcha show with CSS by using the recaptcha AJAX API. In the HTML template I added the following code in the form:
<div class="captcha-container">
<div id="rendered-captcha-container"></div>
</div>
In the client.js file I loaded the script after the template is rendered. Waited for the script to load then created the recaptcha:
Template.myTemplate.rendered = function() {
$.getScript('http://www.google.com/recaptcha/api/js/recaptcha_ajax.js', function() {
Recaptcha.create('add_your_public_key_here', 'rendered-captcha-container', {
theme: 'red',
callback: Recaptcha.focus_response_field
});
});
}