It has to do with your URL paths. Either change the baseurl in your config.toml to baseurl = "https://duck-rabbitside.github.io/" (note the trailing slash), or change the lines in your header.html partial such as href="{{ .Site.BaseURL }}css/highlightjs/monokai.css"> to include a slash between {{ .Site.BaseURL }} and the rest of the path: href="{{ .Site.BaseURL }}/css/highlightjs/monokai.css">.
The basurl was already baseurl = "https://duck-rabbitside.github.io/"
First I changed in the header.html modifing href="{{ .Site.BaseURL }}css/highlightjs/monokai.css"> to href="{{ .Site.BaseURL }}/css/highlightjs/monokai.css">
The I added the slash in all the href inside the header.html file. For example I modified also href="{{ .Site.BaseURL }}apple-touch-icon.png" /> to href="{{ .Site.BaseURL }}/apple-touch-icon.png" />.
It seem that nothing changed.