Added a dark mode version of the animated Ghost logo for onboarding

ref https://linear.app/tryghost/issue/IPC-121/add-dark-mode-version-of-the-animated-ghost-logo

ghost/admin/app/components/dashboard/onboarding-checklist.hbs

@@ -1,8 +1,11 @@
 <div class="gh-onboarding-wrapper" data-test-dashboard="onboarding-checklist">
     <div class="gh-onboarding-header">
-        <video width="80" height="80" loop autoplay muted playsinline preload="metadata" style="width: 80px; height: 80px; margin-bottom: 24px;">
+        <video width="80" height="80" loop autoplay muted playsinline preload="metadata" style="width: 80px; height: 80px; margin-bottom: 24px;" class="gh-onboarding-ghost-logo--light">
             <source src="assets/videos/logo-loader.mp4" type="video/mp4" />
         </video>
+        <video width="80" height="80" loop autoplay muted playsinline preload="metadata" style="width: 80px; height: 80px; margin-bottom: 24px;" class="gh-onboarding-ghost-logo--dark">
+            <source src="assets/videos/logo-loader-dark.mp4" type="video/mp4" />
+        </video>
         <h2 class="gh-canvas-title">{{this.siteUrl}} is ready to go!</h2>
         <p>Let’s set your brand new publication up for success.</p>
     </div>

ghost/admin/app/styles/app-dark.css

@@ -1380,6 +1380,14 @@ Onboarding checklist */
     background: #1c1e21;
 }
 
+.gh-onboarding-ghost-logo--light {
+    display: none;
+}
+
+.gh-onboarding-ghost-logo--dark {
+    display: inline-block;
+}
+
 /* ---------------------------------
 Onboarding checklist: Share publication modal */
 

ghost/admin/app/styles/layouts/dashboard.css

@@ -2771,7 +2771,7 @@ Onboarding checklist */
     flex-direction: column;
     min-height: 100vh;
     position: relative;
-    background: url(img/gradient-bg.png) no-repeat;
+    /* background: url(img/gradient-bg.png) no-repeat; */
     background-size: contain;
     background-position: center;
 }
@@ -2781,6 +2781,14 @@ Onboarding checklist */
     margin-bottom: 40px;
 }
 
+.gh-onboarding-ghost-logo--light {
+    display: inline-block;
+}
+
+.gh-onboarding-ghost-logo--dark {
+    display: none;
+}
+
 .gh-onboarding-header p {
     margin-top: 8px;
     margin-bottom: 0;
@@ -2827,6 +2835,7 @@ Onboarding checklist */
     box-shadow: 0px 1px 0px rgba(17,17,26,0.05), 0px 0px 8px rgba(17,17,26,0.1);
     border-radius: 6px;
     border-bottom: none;
+    transition: none;
 }
 
 .gh-onboarding-item:has(+ .gh-onboarding-item--next) {