feat(theme): implement single-file CSS theming engine with dark, light, high-contrast, and soft themes

frontend/src/themes/theme-dark.css

@@ -0,0 +1,56 @@
+/* ==========================================================================
+   MINDSIFT THEME: DARK MONOCHROME (Standard)
+   Alle Farben, Kontraste und Rundungen zentral in einer einzigen Datei.
+   ========================================================================== */
+
+:root,
+:root[data-theme="dark"] {
+  /* --- 1. AKZENTFARBE --- */
+  --accent-primary: #2563eb;
+  --accent-hover: #3b82f6;
+  --accent-dim: rgba(37, 99, 235, 0.14);
+  --accent-border: rgba(59, 130, 246, 0.45);
+  --text-on-accent: #ffffff;
+
+  /* --- 2. OBERFLÄCHEN & HINTERGRÜNDE --- */
+  --bg-app: #121316;
+  --bg-panel: #1a1c20;
+  --bg-panel-elevated: #22252a;
+  --bg-panel-hover: #2a2e35;
+  --bg-input: #121316;
+  --bg-overlay: rgba(0, 0, 0, 0.72);
+  --bg-code: #0d0e11;
+
+  /* --- 3. TEXT & KONTRASTE --- */
+  --text-main: #f4f4f5;
+  --text-muted: #a1a1aa;
+  --text-faint: #71717a;
+
+  /* --- 4. RAHMEN, LINIEN & SCHATTEN --- */
+  --border-color: rgba(255, 255, 255, 0.09);
+  --border-subtle: rgba(255, 255, 255, 0.05);
+  --border-strong: rgba(255, 255, 255, 0.22);
+  --shadow-panel: 0 10px 30px rgba(0, 0, 0, 0.45);
+  --scrollbar-thumb: rgba(255, 255, 255, 0.14);
+  --scrollbar-thumb-hover: rgba(255, 255, 255, 0.26);
+
+  /* --- 5. STATUS- & SIGNALFARBEN --- */
+  --color-success: #22c55e;
+  --bg-success: rgba(34, 197, 94, 0.12);
+  --border-success: rgba(34, 197, 94, 0.38);
+
+  --color-danger: #ef4444;
+  --bg-danger: rgba(239, 68, 68, 0.12);
+  --border-danger: rgba(239, 68, 68, 0.38);
+
+  --color-warning: #f59e0b;
+  --bg-warning: rgba(245, 158, 11, 0.12);
+  --border-warning: rgba(245, 158, 11, 0.38);
+
+  /* --- 6. RUNDUNGEN (Geometrie) --- */
+  --radius-card: 0px;
+  --radius-btn: 2px;
+  --radius-input: 2px;
+  --radius-checkbox: 0px;
+  --radius-pill: 9999px;
+}