Always use relative_url e.g.:
"{{ '/' | relative_url }}"
instead of the "old way" with site.path / site.baseurl:
"{{ site.path }}/"
Remove all site.path references with relative_url e.g.:
<img src="{{site.path}}/themes/thumbnails/{{theme.thumbnail}}">
to
<img src="{{ '/themes/thumbnails/' | relative_url }}{{theme.thumbnail}}">