You wouldn't need to use a function if you only use it once.

script.user.js

@@ -1,5 +1,5 @@
 // ==UserScript==
-// @name        Snap! Dark Theme
+// @name        DarkSnap
 // @namespace   http://tampermonkey.net/
 // @match       https://snap.berkeley.edu/*
 // @exclude     *://snap.berkeley.edu/snap/*
@@ -9,14 +9,8 @@
 // @description Fork of https://forum.snap.berkeley.edu/t/18272
 // ==/UserScript==
 //
-function isBootstrap5() {
-    // Select all elements with data-bs-theme attribute
-    const elementsWithTheme = document.querySelectorAll('[data-bs-theme]');
-    // Check if there are any such elements
-    return elementsWithTheme.length > 0;
-}
 
-if (isBootstrap5()) {
+if (document.querySelectorAll('[data-bs-theme]').length > 0) {
     // Set the data-bs-theme attribute to "dark"
     // By doing some inspectation I determined a lot of pages use
     // bootstrap 5, and this is how you make bootstrap go in dark mode
@@ -48,4 +42,5 @@ style.innerHTML = `
     color: #212529;
   }
 `;
-document.head.appendChild(style);
+document.head.appendChild(style);
+console.info("Loaded DarkSnap.")