theme-switch-button: init feature (#15)

* thanks to https://feathericons.com for svgs
* can be disabled in site config
   Params:
     disableThemeToggle : true

assets/css/header.css

@@ -16,6 +16,7 @@
 
 .logo,
 .menu {
+    display: flex;
     margin: auto var(--gap);
 }
 
@@ -24,8 +25,21 @@
     font-weight: 700;
 }
 
+.theme-toggle svg{
+    height: 18px;
+    margin: 0 10px;
+}
+
+body.dark #moon{
+    vertical-align: middle;
+    display: none;
+}
+
+body:not(.dark) #sun {
+    display: none;
+}
+
 .menu {
-    display: flex;
     list-style: none;
     word-break: keep-all;
     overflow-x: auto;

layouts/partials/footer.html

@@ -44,4 +44,22 @@
     function menu_on_scroll() {
         localStorage.setItem("menu-scroll-position", document.getElementById('menu').scrollLeft);
     }
-</script>
+</script>
+{{- if not .Site.Params.disableThemeToggle }}
+<script>
+    document.getElementById("theme-toggle").addEventListener("click", () => {
+        if (document.body.className.includes("dark")) {
+            document.body.classList.remove('dark');
+            localStorage.setItem("pref-theme", 'light');
+        } else {
+            document.body.classList.add('dark');
+            localStorage.setItem("pref-theme", 'dark');
+        }
+    })
+</script>
+{{- else -}}
+<!-- case where owner disables theme button after deployment, this resets the stored theme -->
+<script>
+    localStorage.removeItem("pref-theme");
+</script>
+{{- end }}

layouts/partials/header.html

@@ -1,14 +1,46 @@
 {{- if eq .Site.Params.defaultTheme "auto" }}
 <script>
-    if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
+    // load memory
+    if (localStorage.getItem("pref-theme") === "dark") {
         document.body.classList.add('dark');
+    } else if (localStorage.getItem("pref-theme") === "light") {
+        document.body.classList.remove('dark')
+    } else {
+        if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
+            document.body.classList.add('dark');
+        }
     }
 </script>
 {{- end }}
 <header class="header">
     <nav class="nav">
-        <p class="logo"><a href="{{ "" | absLangURL }}">{{ .Site.Title }}</a></p>
-
+        <p class="logo">
+            <a href="{{ "" | absLangURL }}">{{ .Site.Title }}</a>
+            {{- if not .Site.Params.disableThemeToggle }}
+            <span class="theme-toggle">
+                <a id="theme-toggle">
+                    <svg id="moon" xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24"
+                        fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round"
+                        stroke-linejoin="round">
+                        <path d="M21 12.79A9 9 0 1 1 11.21 3 7 7 0 0 0 21 12.79z"></path>
+                    </svg>
+                    <svg id="sun" xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24"
+                        fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round"
+                        stroke-linejoin="round">
+                        <circle cx="12" cy="12" r="5"></circle>
+                        <line x1="12" y1="1" x2="12" y2="3"></line>
+                        <line x1="12" y1="21" x2="12" y2="23"></line>
+                        <line x1="4.22" y1="4.22" x2="5.64" y2="5.64"></line>
+                        <line x1="18.36" y1="18.36" x2="19.78" y2="19.78"></line>
+                        <line x1="1" y1="12" x2="3" y2="12"></line>
+                        <line x1="21" y1="12" x2="23" y2="12"></line>
+                        <line x1="4.22" y1="19.78" x2="5.64" y2="18.36"></line>
+                        <line x1="18.36" y1="5.64" x2="19.78" y2="4.22"></line>
+                    </svg>
+                </a>
+            </span>
+            {{- end}}
+        </p>
         {{- $currentPage := . }}
         <ul class="menu" id="menu" onscroll="menu_on_scroll()">
             {{- range .Site.Menus.main }}