Add theme variables for band editor background and border

Introduce CSS variables for band editor colors in light and dark themes. Update .band-editor to use these variables, enabling automatic adaptation to the active theme.

DbFirst.BlazorWebApp/wwwroot/app.css

@@ -7,9 +7,17 @@ html, body {
     font-size: var(--global-size);
 }
 
+/* Theme-Variablen */
+.app-light {
+    --band-editor-bg: #f8f9fa;
+    --band-editor-border: #dee2e6;
+}
+
 .app-dark {
     background-color: #1b1b1b;
     color: #f1f1f1;
+    --band-editor-bg: #2d2d2d;
+    --band-editor-border: #444444;
 }
 
 a, .btn-link {
@@ -72,9 +80,9 @@ h1:focus {
     padding: 12px;
     margin-top: 4px;
     margin-bottom: 16px;
-    border: 1px solid #dee2e6;
+    border: 1px solid var(--band-editor-border);
     border-radius: 4px;
-    background-color: #f8f9fa;
+    background-color: var(--band-editor-bg);
 }
 
 .band-controls {