How can I theme individual fields?
Inside the views-view-field.tpl.php, you can use the $row variable to get your other fields.
If you're using the Devel module, just send a dsm($row); to get the path to the fields you want to access.

For example I've this inside a views field template, to check if a user have something inside the field "twitter_account" :

$twitter_account = "";
if(isset($row-&gt;field_field_twitter_account[0]['raw']['value'])){
    $twitter_account = $row-&gt;field_field_twitter_account[0]['raw']['value'];
}