What is the intended difference between site.url and site.baseurl? The example in _config.yml is:

# This URL is the main address for absolute links. Don't include a slash at the end.
#
url: 'https://phlow.github.io/feeling-responsive'
baseurl: ''

But in the footer, site.baseurl is the prefix for items without "http" in the URL, which results in that link being broken (I'm seeing links to "http://phlow.github.io/feed.xml" on your demo, a 404). There are also many instances of site.url and site.baseurl being put together, like this:

{{ site.url }}{{ site.baseurl }}/assets/img/{{ site.logo }}

My best guess is that site.url is supposed to be the domain name, and site.baseurl is supposed to be the root directory of the site (if any). So the example page should actually be:

url: 'https://phlow.github.io'
baseurl: '/feeling-responsive'

This way, an absolute link is {{ site.url }}{{ site.baseurl }}, and a relative link is just {{ site.baseurl }}.

After I set my config file like this, and then went through all the code I could find using these variables and adjusted them accordingly, I stopped getting broken links. I'm not too familiar with Jekyll, though, so I'm not sure if this structure is correct, or unusual, or breaks a standard convention.

Edit: That is to say, I made this change and then different stuff was broken :) But I was able to fix the new broken links, etc. and get my site working properly as far as I can tell.