dd a class="" to anchor on the "read more" links of a node
Add template.php file to your theme directory if it doesn't exist. Then, add template_preprocess_node() to it like so, replacing template with your theme name.

function template_preprocess_node(&amp;$variables) {

  $variables['links']['node']['#links']['node-readmore']['attributes']['class'][] = 'btn-default';
}