theme: add accent color

layouts/partials/head.html

@@ -2,6 +2,13 @@
     <meta charset="utf-8">
     <meta http-equiv="X-UA-Compatible" content="IE=edge">
     <meta name="viewport" content="width=device-width, initial-scale=1">
+
+    <style>
+        :root {
+            --accent-color: {{ .Site.Params.AccentColor | default "#E66100" }};
+        }
+    </style>
+
     <link rel="stylesheet" type="text/css" href="/css/style.css">
 
     {{ $title := .Title | default .Site.Title }}