fix: hardcode theme CSS path in head.html (site.theme removed)

Author: WangSimiao2000

_includes/head.html

@@ -87,7 +87,7 @@
   {% endunless %}
 
   <!-- Theme style -->
-  <link rel="stylesheet" href="{{ '/assets/css/:THEME.css' | replace: ':THEME', site.theme | relative_url }}">
+  <link rel="stylesheet" href="{{ '/assets/css/jekyll-theme-chirpy.css' | relative_url }}">
 
   <!-- Web Font -->
   <link rel="stylesheet" href="{{ site.data.origin[type].webfonts | relative_url }}">