How add class the_content();?

You don’t. You’ll need to put an element around it:

<?php if ( $content_source == 'excerpt' ) { ?>
    <div class="excerpt">
        <?php the_excerpt(); ?>
    </div>
<?php } else { ?>
    <div class="content">
        <?php the_content(); ?>
    </div>
<?php } ?> 

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)