This change can’t be made in custom.css file.
You can edit the title in wp-content/themes/legenda/woocommerce/single-product/up-sells.php file
You will find the code:
etheme_create_flex_slider($args, __('Our offers', ETHEME_DOMAIN), false, true);
and change “Our offers” title to anything you want.
One standard license is valid only for 1 project. Running multiple projects on a single license is a copyright violation.