fix: Preview — white default bg, black text, dark borders on tables

Views/Slides/Preview.cshtml

@@ -1,7 +1,7 @@
 @model Slide
 @{
     Layout = null;
-    var bgStyle = $"background: {Model.BackgroundColor ?? "#1a1a2e"};";
+    var bgStyle = $"background: {Model.BackgroundColor ?? "#ffffff"};";
     if (!string.IsNullOrEmpty(Model.BackgroundImage))
     {
         bgStyle += $" background-image: url('{Model.BackgroundImage}'); background-size: {Model.BackgroundSize ?? "cover"}; background-position: center; background-repeat: no-repeat;";
@@ -18,7 +18,7 @@
         html, body { width: 100%; height: 100%; overflow: hidden; }
         body {
             @Html.Raw(bgStyle)
-            color: #fff;
+            color: #000;
             font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
         }
         .slide-content {
@@ -28,11 +28,11 @@
         }
         .slide-content img { max-width: 100%; height: auto; }
         .slide-content table { border-collapse: collapse; }
-        .slide-content table td, .slide-content table th { border: 1px solid rgba(255,255,255,0.3); padding: 8px 12px; }
+        .slide-content table td, .slide-content table th { border: 1px solid rgba(0,0,0,0.2); padding: 8px 12px; }
         iframe.embed-frame { width: 100%; height: 100%; border: none; }
         .ics-events { padding: 2em; }
         .ics-events h2 { margin-bottom: 1em; font-size: 2em; }
-        .event-card { background: rgba(255,255,255,0.1); border-radius: 12px; padding: 1.2em; margin-bottom: 1em; }
+        .event-card { background: rgba(0,0,0,0.05); border-radius: 12px; padding: 1.2em; margin-bottom: 1em; }
         .event-card h3 { font-size: 1.3em; margin-bottom: 0.3em; }
         .event-meta { opacity: 0.7; font-size: 0.9em; }
         .back-link { position: fixed; top: 10px; right: 10px; z-index: 999; }