The newer versions of our Tumblr themes do not support custom inputs for your custom links. Some users dive it into the theme code, and modify the code to create links. We do not recommend this practice as it may break your theme view. Instead, we highly recommend that you use the pre-built 'Add a page' option on your theme customize page. It may be awkward to use it as a theme link, but it may be better to separate your links from options.
The screenshot below may help you locate the button. It's right there. If you haven't been using the custom pages feature on Tumblr, we highly recommend that you take time, and look through it. You can create a variety of looks with this wonderful Tumblr feature.
Tumblr custom pages support three different page types. One is 'Standard layout', which is basically just a page pre-built by the theme. It works just like a text post. The other type 'Custom layout' is useful if you are building a custom page off from the theme. When you open it, you'd see a blank page with no theme elements on it – it's normal. You are basically building a page without any styles applied.
The last thing is 'Custom link'. You may have external links such as your Instagram page, twitter profile, or store. Assign there. The newer versions of our Tumblr themes automatically detect the types of the links, and create icons, or links on your theme.
Hope this helps!