✨ feat: change default theme to OS default

Update the theme switcher code to set the default theme to the OS
default, rather than the "light" theme. This provides a more seamless
user experience for users whose OS is set to a dark mode. The current
theme setting in local storage will still be respected if present. This
change was made to improve the accessibility and usability.

Fixes #38

static/js/main.js

@@ -1,14 +1,15 @@
-//https://codepen.io/codeorum/pen/bGedRJO
-
 var themeSwitcher = document.querySelector('.theme-switcher input');
 var currentTheme = localStorage.getItem('theme');
 
-// check what is current theme right now and activate it
+// detect the user's preferred color scheme and activate it
 if (currentTheme) {
     document.documentElement.setAttribute('data-theme', currentTheme);
     if (currentTheme === 'dark') {
         themeSwitcher.checked = true;
     }
+} else if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
+    document.documentElement.setAttribute('data-theme', 'dark');
+    themeSwitcher.checked = true;
 }
 
 // switch between themes