theme-switch-button: improve logic

- owner can set dark/light/auto default theme
- theme toggle will be enabled on all theme modes unless disbaled
- if disableThemeToggle is true, user won't be able to switch themes,
here auto (a/c to system theme) also works

for #69

layouts/partials/footer.html

@@ -50,7 +50,7 @@
         localStorage.setItem("menu-scroll-position", document.getElementById('menu').scrollLeft);
     }
 </script>
-{{- if (and (not .Site.Params.disableThemeToggle) (not (or (eq .Site.Params.defaultTheme "light") (eq .Site.Params.defaultTheme "dark")))) }}
+{{- if (and (not .Site.Params.disableThemeToggle) (not (eq .Site.Params.defaultTheme "auto"))) }}
 <script>
     document.getElementById("theme-toggle").addEventListener("click", () => {
         if (document.body.className.includes("dark")) {

layouts/partials/header.html

@@ -1,4 +1,4 @@
-{{- if (and (not .Site.Params.disableThemeToggle) (not (or (eq .Site.Params.defaultTheme "light") (eq .Site.Params.defaultTheme "dark")))) }}
+{{- if (and (not .Site.Params.disableThemeToggle) (not (eq .Site.Params.defaultTheme "auto"))) }}
 <script>
     // load memory
     if (localStorage.getItem("pref-theme") === "dark") {
@@ -13,7 +13,7 @@
 
 </script>
 {{- end }}
-{{- if (and (.Site.Params.disableThemeToggle) (eq .Site.Params.defaultTheme "auto")) }}
+{{- if (eq .Site.Params.defaultTheme "auto") }}
 <script>
     if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
         document.body.classList.add('dark');
@@ -41,7 +41,7 @@
                 {{- .Site.Params.label.text | default .Site.Title -}}
             </a>
             <span class="logo-switches">
-                {{- if (and (not .Site.Params.disableThemeToggle) (not (or (eq .Site.Params.defaultTheme "light") (eq .Site.Params.defaultTheme "dark")))) }}
+                {{- if (not .Site.Params.disableThemeToggle) }}
                 <span class="theme-toggle">
                     <a id="theme-toggle" accesskey="t">
                         <svg id="moon" xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24"