Allowed a site-specific default Neon theme

header-stylesheets.html

@@ -2,7 +2,7 @@
 <link rel="stylesheet" type="text/css" id="neon-tables"          href="https://stylesheets.bhstudios.org/assets/v2/neon/neon-tables.css" />
 <link rel="stylesheet" type="text/css" id="stylesheet-print"     href="https://stylesheets.bhstudios.org/assets/v2/basics/print.css" />
 <link rel="stylesheet" type="text/css" id="framework-stylesheet" href="https://stylesheets.bhstudios.org/assets/v2/neon/neon-renderer.css" />
-<link rel="stylesheet" type="text/css" id="theme-stylesheet"     href="https://stylesheets.bhstudios.org/assets/v2/neon/neon-water.css" />
+<link rel="stylesheet" type="text/css" id="theme-stylesheet"     href="{{ site.default-neon-theme-url | default: 'https://stylesheets.bhstudios.org/assets/v2/neon/neon-water.css' }}" />
 <link rel="stylesheet" type="text/css" id="material-icons"       href="https://fonts.googleapis.com/icon?family=Material+Icons" />
 <link rel="stylesheet" type="text/css" id="google-fonts"         href="https://fonts.googleapis.com/css?family=Roboto:100,100i,300,300i,400,400i,500,500i,700|Questrial|ZCOOL+QingKe+HuangYou|Oswald:600&amp;subset=chinese-simplified,korean,latin-ext,vietnamese,cyrillic,cyrillic-ext,greek,greek-ext">
 <link rel="stylesheet" type="text/css" id="site-styles"          href="{{ site.baseurl }}/site.css" />