website: fix light-dark themes switch

Reviewed by: carlavilla, ziaee
Approved by: carlavilla
Differential Revision: https://reviews.freebsd.org/D57057
PR: 295341

website/themes/beastie/assets/js/theme-chooser.js

@@ -0,0 +1,47 @@
+/*
+BSD 2-Clause License
+
+Copyright (c) 1994-2026, The FreeBSD Documentation Project
+Copyright (c) 2026,      Vladlen Popolitov
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+   list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*/
+
+// FreeBSD light/dark theme persists
+;(function() {
+    const toggle = document.getElementById('theme-switch');
+    if (!toggle) return;
+
+    // restore saved state
+    const saved = localStorage.getItem('theme-preference');
+    if (saved === 'dark') {
+    toggle.checked = true;
+    } else if (saved === 'light') {
+    toggle.checked = false;
+    }
+    // if state was no saved - use current state (CSS uses prefers-color-scheme)
+    // save state, when theme switched to or from dark
+    toggle.addEventListener('change', function() {
+    localStorage.setItem('theme-preference', this.checked ? 'dark' : 'light');
+    });
+})();

website/themes/beastie/layouts/_partials/site-head.html

@@ -44,6 +44,9 @@
   </script>
   <!-- END SEO -->
 
+  {{- $themeChooser := resources.Get "js/theme-chooser.js" | resources.Minify }}
+  <script defer src="{{ $themeChooser.RelPermalink }}"></script>
+
   {{- if hugo.IsServer -}}
     {{- printf "<!-- Dev mode: Not loading plausible script to avoid bloating stats -->" | safeHTML }}
   {{- else -}}