By description tag you mean the meta description on the
<head> right? That can’t be added to the theme since that is a plugin territory functionality and is against wordpress.org’s theme guidelines. You can simply use any SEO plugin like Yoast.
For the alt value on the logo, I’ve already added a default value for the next update. If you want more control you can either edit header.php or disable the logo and add a Text Widget on your header area where you can add your own logo markup.