diff --git a/src/js/settings.js b/src/js/settings.js
index a15e969..5bfdefc 100644
--- a/src/js/settings.js
+++ b/src/js/settings.js
@@ -116,6 +116,94 @@ function isValidBgUrl(url) {
     (url.startsWith('blob:') || url.startsWith('data:image/') || url.startsWith('https://'));
 }
 
+// ---- THEME-BUILDER: Konstanten + reine Helfer ----
+const CUSTOM_DEFAULTS = {
+  accent: '#6c8cff', bgPrimary: '#0b0d12', bgBoard: '#141821',
+  textPrimary: '#e6e8ef', textSecondary: '#9aa3b8', textMuted: '#5b6478',
+};
+const HEX_RE = /^#([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$/;
+
+function isValidHexColor(v) { return typeof v === 'string' && HEX_RE.test(v); }
+function safeHex(v, fallback) { return isValidHexColor(v) ? v : fallback; }
+
+function hexToRgba(hex, alpha) {
+  let h = hex.replace('#', '');
+  if (h.length === 3) h = h.split('').map(c => c + c).join('');
+  const r = parseInt(h.slice(0, 2), 16), g = parseInt(h.slice(2, 4), 16), b = parseInt(h.slice(4, 6), 16);
+  return `rgba(${r}, ${g}, ${b}, ${alpha})`;
+}
+
+// WCAG 2.x Relativluminanz + Kontrastverhaeltnis
+function relLuminance(hex) {
+  let h = hex.replace('#', '');
+  if (h.length === 3) h = h.split('').map(c => c + c).join('');
+  const lin = [0, 2, 4].map(i => {
+    const c = parseInt(h.slice(i, i + 2), 16) / 255;
+    return c <= 0.03928 ? c / 12.92 : Math.pow((c + 0.055) / 1.055, 2.4);
+  });
+  return 0.2126 * lin[0] + 0.7152 * lin[1] + 0.0722 * lin[2];
+}
+function contrastRatio(hexA, hexB) {
+  const a = relLuminance(hexA), b = relLuminance(hexB);
+  return (Math.max(a, b) + 0.05) / (Math.min(a, b) + 0.05);
+}
+
+function updateContrastIndicator(textHex, bgHex) {
+  const el = document.getElementById('tbContrast');
+  if (!el) return;
+  const ratio = contrastRatio(textHex, bgHex);
+  let cls, key;
+  if (ratio >= 4.5) { cls = 'good'; key = 'theme.builder.contrast_good'; }
+  else if (ratio >= 3) { cls = 'ok'; key = 'theme.builder.contrast_ok'; }
+  else { cls = 'bad'; key = 'theme.builder.contrast_bad'; }
+  el.classList.remove('good', 'ok', 'bad');
+  el.classList.add(cls);
+  const txt = document.getElementById('tbContrastText');
+  if (txt) txt.textContent = `${t(key)} (${ratio.toFixed(1)}:1)`;
+}
+
+// Setzt data-theme='custom' + 6 validierte Inline-Vars (Gate vor jedem setProperty).
+function applyCustomTheme(ct) {
+  const root = document.documentElement;
+  const c = ct || {};
+  const accent        = safeHex(c.accent,        CUSTOM_DEFAULTS.accent);
+  const bgPrimary     = safeHex(c.bgPrimary,     CUSTOM_DEFAULTS.bgPrimary);
+  const bgBoard       = safeHex(c.bgBoard,       CUSTOM_DEFAULTS.bgBoard);
+  const textPrimary   = safeHex(c.textPrimary,   CUSTOM_DEFAULTS.textPrimary);
+  const textSecondary = safeHex(c.textSecondary, CUSTOM_DEFAULTS.textSecondary);
+  const textMuted     = safeHex(c.textMuted,     CUSTOM_DEFAULTS.textMuted);
+
+  root.setAttribute('data-theme', 'custom');
+  root.style.setProperty('--accent', accent);
+  root.style.setProperty('--bg-primary', bgPrimary);
+  root.style.setProperty('--bg-board', hexToRgba(bgBoard, 0.55));
+  root.style.setProperty('--text-primary', textPrimary);
+  root.style.setProperty('--text-secondary', textSecondary);
+  root.style.setProperty('--text-muted', textMuted);
+
+  document.querySelectorAll('.theme-card').forEach(card => {
+    const on = card.dataset.value === 'custom';
+    card.classList.toggle('active', on);
+    card.setAttribute('aria-pressed', on ? 'true' : 'false');
+  });
+  updateContrastIndicator(textPrimary, bgPrimary);
+}
+
+// Entfernt die 6 Inline-Vars (Rueckwechsel auf Preset / Reset).
+function clearCustomTheme() {
+  const root = document.documentElement;
+  ['--accent', '--bg-primary', '--bg-board', '--text-primary', '--text-secondary', '--text-muted']
+    .forEach(v => root.style.removeProperty(v));
+}
+
+// Schreibt die gespeicherten (oder Default-) Farben in die 6 Picker-Inputs.
+function syncCustomPickers() {
+  const ct = settings.customTheme || {};
+  const set = (id, key) => { const el = document.getElementById(id); if (el) el.value = safeHex(ct[key], CUSTOM_DEFAULTS[key]); };
+  set('tbAccent', 'accent'); set('tbBg', 'bgPrimary'); set('tbBoard', 'bgBoard');
+  set('tbText', 'textPrimary'); set('tbTextSec', 'textSecondary'); set('tbTextMuted', 'textMuted');
+}
+
 // Eigenes Upload-Bild Quota-schonend verkleinern: auf die laengste Bildschirmkante
 // (× devicePixelRatio, gedeckelt) herunterrechnen und als WebP neu kodieren. Das spart
 // gegenueber dem rohen Base64-Upload locker den Grossteil der chrome.storage.local-Quota.