Fixed CI error, made theme persistent

When you reloaded the page in dark mode, it would flash white. This shouldn't happen anymore. Additionally, the no src error should be eliminated.

Author: JManion32

_layouts/default.html

@@ -7,7 +7,6 @@
         <meta name="generator" content="Jekyll v{{ jekyll.version }}">
 
         <script>
-        document.addEventListener('DOMContentLoaded', () => {
             const m = localStorage.getItem('site_mode') || 'light';
             document.documentElement.setAttribute('data-theme', m);
             const logo = document.getElementById('site-logo');
@@ -16,7 +15,6 @@
                 ? logo.dataset.dark
                 : logo.dataset.light;
             }
-        });
         </script>
 
         <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Merriweather:400,400italic,700,700italic|Open+Sans:400,400italic,600,600italic,700,700italic|Inconsolata:400,700">
@@ -87,11 +85,11 @@ <h1>
             </script>
             <div class="dark-mode-container mobile-invisible mobile-toggle">
                 <label class="dark-mode-switch">
-                    <input type="checkbox" id="dark-mode-toggle" />
+                <input type="checkbox" id="dark-mode-toggle" />
                     <span class="slider">
-                        <!-- actual image instead of pseudo-element -->
                         <img
                         id="dark-mode-icon"
+                        src="/images/sun_black.png"
                         data-light="/images/sun_black.png"
                         data-dark="/images/moon_white.png"
                         alt="mode icon"