refactor: update toggle theme function for better structure and a11y

Move theme switch js codes in Layout.astro file to external js file (toggle-theme.js). Update
.prettierignore to exclude the new toggle-theme.js file.  Update base.css for new data-theme
attribute.

public/toggle-theme.js

@@ -0,0 +1,52 @@
+const primaryColorScheme = ""; // "light" | "dark"
+
+// Get theme data from local storage
+const currentTheme = localStorage.getItem("theme");
+
+function getPreferTheme() {
+  // return theme value in local storage if it is set
+  if (currentTheme) return currentTheme;
+
+  // return primary color scheme if it is set
+  if (primaryColorScheme) return primaryColorScheme;
+
+  // return user device's prefer color scheme
+  return window.matchMedia("(prefers-color-scheme: dark)").matches
+    ? "dark"
+    : "light";
+}
+
+let themeValue = getPreferTheme();
+
+function setPreference() {
+  localStorage.setItem("theme", themeValue);
+  reflectPreference();
+}
+
+function reflectPreference() {
+  document.firstElementChild.setAttribute("data-theme", themeValue);
+
+  document.querySelector("#theme-btn")?.setAttribute("aria-label", themeValue);
+}
+
+// set early so no page flashes / CSS is made aware
+reflectPreference();
+
+window.onload = () => {
+  // set on load so screen readers can get the latest value on the button
+  reflectPreference();
+
+  // now this script can find and listen for clicks on the control
+  document.querySelector("#theme-btn").addEventListener("click", () => {
+    themeValue = themeValue === "light" ? "dark" : "light";
+    setPreference();
+  });
+};
+
+// sync with system changes
+window
+  .matchMedia("(prefers-color-scheme: dark)")
+  .addEventListener("change", ({ matches: isDark }) => {
+    themeValue = isDark ? "dark" : "light";
+    setPreference();
+  });