diff --git a/Web/static/css/styles.css b/Web/static/css/styles.css
index d624b20..1346daf 100755
--- a/Web/static/css/styles.css
+++ b/Web/static/css/styles.css
@@ -31,6 +31,9 @@
     --ui-text: #f8fafc;
     --ui-text-muted: #94a3b8;
     --ui-title: #ffffff;
+    --ui-overlay: #020617;
+    --ui-overlay-surface: #1e293b;
+    --ui-overlay-border: #64748b;
     --ui-shadow-sm: 0 4px 12px rgba(0, 0, 0, 0.4), inset 0 0 0 1px rgba(255, 255, 255, 0.05);
     --ui-shadow-md: 0 10px 30px rgba(0, 0, 0, 0.6), inset 0 0 0 1px rgba(255, 255, 255, 0.05);
     
@@ -39,6 +42,10 @@
     --ui-btn-radius: 99px; /* Pillenform für Buttons */
 }
 
+:root[data-theme="dark"] {
+    color-scheme: dark;
+}
+
 /* Safe Area Support for iPad notches, Dynamic Island, and rounded corners */
 html {
     /* Ensure viewport-fit is respected */
@@ -1002,3 +1009,104 @@ html, body {
 :root[data-theme="dark"] .favorite-item {
     outline: 2px solid #f59e0b !important;
 }
+
+/* Keep overlays and dialogs sufficiently opaque for readable content. */
+:root[data-theme="dark"] .modal,
+:root[data-theme="dark"] .modal-backdrop,
+:root[data-theme="dark"] .ui-modal-overlay,
+:root[data-theme="dark"] .item-modal,
+:root[data-theme="dark"] .reset-modal-overlay,
+:root[data-theme="dark"] #onboarding-overlay,
+:root[data-theme="dark"] [id$="-modal"][style*="inset"],
+:root[data-theme="dark"] [id$="Modal"][style*="inset"] {
+    background-color: var(--ui-overlay) !important;
+}
+
+:root[data-theme="dark"] .modal-content,
+:root[data-theme="dark"] .ui-modal-surface,
+:root[data-theme="dark"] .reset-modal-container,
+:root[data-theme="dark"] #onboarding-modal,
+:root[data-theme="dark"] [id$="-modal"] > div,
+:root[data-theme="dark"] [id$="Modal"] > div {
+    background-color: var(--ui-overlay-surface) !important;
+    color: var(--ui-text) !important;
+    border-color: var(--ui-overlay-border) !important;
+}
+
+/* Inline light surfaces must not become translucent or low-contrast in dark mode. */
+:root[data-theme="dark"] [style*="background: white"],
+:root[data-theme="dark"] [style*="background:#fff"],
+:root[data-theme="dark"] [style*="background: #fff"],
+:root[data-theme="dark"] [style*="background-color: white"],
+:root[data-theme="dark"] [style*="background-color:#fff"],
+:root[data-theme="dark"] [style*="background-color: #fff"] {
+    background-color: var(--ui-surface) !important;
+    color: var(--ui-text) !important;
+}
+
+/* Shared dark-mode surfaces for every template that extends base.html. */
+:root[data-theme="dark"] .container,
+:root[data-theme="dark"] .card,
+:root[data-theme="dark"] .card-body,
+:root[data-theme="dark"] .card-header,
+:root[data-theme="dark"] .card-footer,
+:root[data-theme="dark"] .content,
+:root[data-theme="dark"] .form-card,
+:root[data-theme="dark"] .form-container,
+:root[data-theme="dark"] .upload-container,
+:root[data-theme="dark"] .edit-container,
+:root[data-theme="dark"] .admin-content-container,
+:root[data-theme="dark"] .calendar-container,
+:root[data-theme="dark"] .table-container,
+:root[data-theme="dark"] .notice-card,
+:root[data-theme="dark"] .settings-card,
+:root[data-theme="dark"] .summary-card,
+:root[data-theme="dark"] .invoice-card,
+:root[data-theme="dark"] .student-card-form,
+:root[data-theme="dark"] .user-management-container,
+:root[data-theme="dark"] .error-container,
+:root[data-theme="dark"] .library-filter-panel,
+:root[data-theme="dark"] .library-items-table,
+:root[data-theme="dark"] .info-box {
+    background-color: var(--ui-surface) !important;
+    color: var(--ui-text) !important;
+    border-color: var(--ui-border) !important;
+}
+
+:root[data-theme="dark"] .table,
+:root[data-theme="dark"] .table-responsive,
+:root[data-theme="dark"] .table > :not(caption) > *,
+:root[data-theme="dark"] .table > :not(caption) > * > *,
+:root[data-theme="dark"] .library-items-table td,
+:root[data-theme="dark"] .library-items-table th {
+    background-color: var(--ui-surface) !important;
+    color: var(--ui-text) !important;
+    border-color: var(--ui-border) !important;
+}
+
+:root[data-theme="dark"] .table-striped > tbody > tr:nth-of-type(odd) > *,
+:root[data-theme="dark"] .table-light,
+:root[data-theme="dark"] .table-light > *,
+:root[data-theme="dark"] .bg-white,
+:root[data-theme="dark"] .bg-light,
+:root[data-theme="dark"] .bg-body,
+:root[data-theme="dark"] .bg-body-secondary {
+    background-color: var(--ui-surface-soft) !important;
+    color: var(--ui-text) !important;
+}
+
+:root[data-theme="dark"] .form-text,
+:root[data-theme="dark"] .text-muted,
+:root[data-theme="dark"] small,
+:root[data-theme="dark"] .library-load-hint {
+    color: var(--ui-text-muted) !important;
+}
+
+/* White translucent template decorations become stable, readable surfaces. */
+:root[data-theme="dark"] [style*="background: rgba(255,255,255"],
+:root[data-theme="dark"] [style*="background: rgba(255, 255, 255"],
+:root[data-theme="dark"] [style*="background-color: rgba(255,255,255"],
+:root[data-theme="dark"] [style*="background-color: rgba(255, 255, 255"] {
+    background-color: var(--ui-surface-soft) !important;
+    color: var(--ui-text) !important;
+}
diff --git a/Web/templates/base.html b/Web/templates/base.html
index 174204b..007405e 100755
--- a/Web/templates/base.html
+++ b/Web/templates/base.html
@@ -2396,5 +2396,151 @@
         })();
     </script>
 
+    <style id="dark-mode-component-overrides">
+        :root[data-theme="dark"] .mahnungen-head,
+        :root[data-theme="dark"] .mahnungen-card,
+        :root[data-theme="dark"] .tutorial-side,
+        :root[data-theme="dark"] .workflow-step,
+        :root[data-theme="dark"] .tutorial-note,
+        :root[data-theme="dark"] .tutorial-tooltip-toggle,
+        :root[data-theme="dark"] .bulk-delete-drawer,
+        :root[data-theme="dark"] .filter-group,
+        :root[data-theme="dark"] .filter-dropdown,
+        :root[data-theme="dark"] .calendar-wrapper,
+        :root[data-theme="dark"] .calendar-day-details,
+        :root[data-theme="dark"] .cal-booking-item,
+        :root[data-theme="dark"] .edit-form,
+        :root[data-theme="dark"] .password-change-form,
+        :root[data-theme="dark"] .upload-form,
+        :root[data-theme="dark"] .book-info-container,
+        :root[data-theme="dark"] .student-card-form,
+        :root[data-theme="dark"] .user-management-container {
+            background: var(--ui-surface) !important;
+            background-color: var(--ui-surface) !important;
+            color: var(--ui-text) !important;
+            border-color: var(--ui-border) !important;
+        }
+
+        :root[data-theme="dark"] .mahnungen-table th,
+        :root[data-theme="dark"] .calendar-header,
+        :root[data-theme="dark"] .filter-inputs,
+        :root[data-theme="dark"] .items-loading-indicator,
+        :root[data-theme="dark"] .detail-group.full-width .detail-value,
+        :root[data-theme="dark"] .library-filter-panel,
+        :root[data-theme="dark"] .card-header,
+        :root[data-theme="dark"] .modal-header,
+        :root[data-theme="dark"] .modal-footer {
+            background: var(--ui-surface-soft) !important;
+            background-color: var(--ui-surface-soft) !important;
+            color: var(--ui-text) !important;
+            border-color: var(--ui-border) !important;
+        }
+
+        :root[data-theme="dark"] .mahnungen-table tr:hover td,
+        :root[data-theme="dark"] .filter-option:hover,
+        :root[data-theme="dark"] .calendar-header button:hover,
+        :root[data-theme="dark"] .workflow-nav button:hover {
+            background: var(--ui-bg-accent) !important;
+            background-color: var(--ui-bg-accent) !important;
+        }
+
+        :root[data-theme="dark"] .mahnungen-head h1,
+        :root[data-theme="dark"] .tutorial-side h2,
+        :root[data-theme="dark"] .workflow-step h3,
+        :root[data-theme="dark"] .calendar-header span,
+        :root[data-theme="dark"] .calendar-day-details h5,
+        :root[data-theme="dark"] .detail-value,
+        :root[data-theme="dark"] .cal-booking-time,
+        :root[data-theme="dark"] .cal-booking-user,
+        :root[data-theme="dark"] .cal-booking-note,
+        :root[data-theme="dark"] .form-group label,
+        :root[data-theme="dark"] .page-header h1,
+        :root[data-theme="dark"] .card-header h2 {
+            color: var(--ui-title) !important;
+        }
+
+        :root[data-theme="dark"] .mahnungen-head p,
+        :root[data-theme="dark"] .tutorial-side p,
+        :root[data-theme="dark"] .workflow-step p,
+        :root[data-theme="dark"] .workflow-step li,
+        :root[data-theme="dark"] .tutorial-tooltip-toggle label,
+        :root[data-theme="dark"] .tooltip-description,
+        :root[data-theme="dark"] .page-header p,
+        :root[data-theme="dark"] .form-group small,
+        :root[data-theme="dark"] .cal-booking-label,
+        :root[data-theme="dark"] .cal-empty-state {
+            color: var(--ui-text-muted) !important;
+        }
+
+        :root[data-theme="dark"] .workflow-nav button,
+        :root[data-theme="dark"] .bulk-delete-actions button,
+        :root[data-theme="dark"] .filter-toggle,
+        :root[data-theme="dark"] .clear-filter {
+            background: var(--ui-surface-soft) !important;
+            background-color: var(--ui-surface-soft) !important;
+            color: var(--ui-text) !important;
+            border-color: var(--ui-border) !important;
+        }
+
+        :root[data-theme="dark"] .workflow-nav button.active,
+        :root[data-theme="dark"] .filter-toggle.is-open {
+            background: #1d4ed8 !important;
+            color: #ffffff !important;
+            border-color: #60a5fa !important;
+        }
+
+        :root[data-theme="dark"] .badge-light,
+        :root[data-theme="dark"] .badge.bg-light,
+        :root[data-theme="dark"] .badge.bg-secondary,
+        :root[data-theme="dark"] .filter-tag,
+        :root[data-theme="dark"] .tutorial-pill {
+            background: var(--ui-surface-soft) !important;
+            color: var(--ui-text) !important;
+            border-color: var(--ui-border) !important;
+        }
+
+        :root[data-theme="dark"] .form-control,
+        :root[data-theme="dark"] .form-select,
+        :root[data-theme="dark"] input,
+        :root[data-theme="dark"] select,
+        :root[data-theme="dark"] textarea {
+            background-color: var(--ui-bg) !important;
+            color: var(--ui-text) !important;
+            border-color: var(--ui-border) !important;
+        }
+
+        :root[data-theme="dark"] ::placeholder {
+            color: #b7c4d6 !important;
+            opacity: 1 !important;
+        }
+
+        :root[data-theme="dark"] .modal-content,
+        :root[data-theme="dark"] .modal-dialog-white,
+        :root[data-theme="dark"] .modal-body,
+        :root[data-theme="dark"] .modal-content-margin {
+            background: var(--ui-overlay-surface) !important;
+            background-color: var(--ui-overlay-surface) !important;
+            color: var(--ui-text) !important;
+            border-color: var(--ui-overlay-border) !important;
+        }
+
+        :root[data-theme="dark"] .modal-content h1,
+        :root[data-theme="dark"] .modal-content h2,
+        :root[data-theme="dark"] .modal-content h3,
+        :root[data-theme="dark"] .modal-content h4,
+        :root[data-theme="dark"] .modal-content h5,
+        :root[data-theme="dark"] .modal-content h6,
+        :root[data-theme="dark"] .modal-content label,
+        :root[data-theme="dark"] .modal-content p,
+        :root[data-theme="dark"] .modal-content span {
+            color: var(--ui-text) !important;
+        }
+
+        :root[data-theme="dark"] .guest-impressum-footer a {
+            color: #dbeafe !important;
+            border-bottom-color: #93c5fd !important;
+        }
+    </style>
+
 </body>
 </html>
diff --git a/Web/templates/library_table.html b/Web/templates/library_table.html
index 953d3f0..ad024e9 100644
--- a/Web/templates/library_table.html
+++ b/Web/templates/library_table.html
@@ -650,6 +650,7 @@
     // =========================================================================
     function createModalOverlay() {
         const overlay = document.createElement('div');
+        overlay.className = 'ui-modal-overlay';
         overlay.style.position = 'fixed';
         overlay.style.top = '0';
         overlay.style.left = '0';
@@ -665,6 +666,7 @@
 
     function createModalBox() {
         const box = document.createElement('div');
+        box.className = 'ui-modal-surface';
         box.style.backgroundColor = '#fff';
         box.style.padding = '24px';
         box.style.borderRadius = '8px';