add variaable for header bg class

layouts/partials/page-header.html

@@ -14,7 +14,7 @@
   </header>
 {{ else }}
   <header>
-    <div class="bg-near-blackTK bg-black">
+    <div class="{{ .Site.Params.background_color_class | default "bg-black" }}">
       {{ partial "site-navigation.html" . }}
     </div>
   </header>

layouts/partials/site-header.html

@@ -16,7 +16,7 @@
   </header>
 {{ else }}
   <header>
-    <div class="pb3-m pb6-l bg-black">
+    <div class="pb3-m pb6-l {{ .Site.Params.background_color_class | default "bg-black" }}">
       {{ partial "site-navigation.html" . }}
       <div class="tc-l pv3 ph3 ph4-ns">
         <h1 class="f2 f-subheadline-l fw2 light-silver mb0 lh-title">