There is one little extra option you can use if you know CSS. Once again open up Customize page and now open up Advanced section. The last box “Add custom CSS” gives you the option to modify the CSS from Tumblr Customize page so you don’t have to modify the theme file directly.
You can also create Custom Pages inside Tumblr Dashboard. Just click Pages when you are on Customize page and then you can create as many as you like.