There are some cool features Fine Line offers that might not be immediatley apparent, so please read this quick guide before you use it!
Fine Line allows you to pick from hundres of fonts via Google Web Fonts. Just pick any font you want from their website, and type it's name, exactly as it appears on Google Fonts, in the text box labelled Google Web Font Name.
Social network support
This theme also supports most major social networks. All you need to do is type your username in the relevant box. You only need to put the bit after the domain name, i.e. facebook.com/john_madden or twitter.com/horse_ebooks.
For Google+, you can get your ID by going your profile, and it's the long numbers in the URL bar, i.e. plus.google.com/115734744988219511518/posts. Nobody really uses Google+ anyway, you could just not bother...
If you untick the Use mobile optimized layout option in the Advanced section of the Customize page, this theme will be what users see when they look at your blog on their mobile device (iPad, iPhone, Android thing, etc). This is preferable to Tumblr's default theme, because mobile users see all your links, and it creates a consistent experience.