@@ -115,6 +115,99 @@ function buildGraphologyGraph(dot) {
 // Popup helpers (referenced by onclick attributes in the report template).
 // ---------------------------------------------------------------------------
 
+// Hovered-node label box backgrounds per palette. Sigma's default hover
+// renderer hardcodes "#FFF", which washes out the light label text the dark
+// palette switches to; the dark value is --card-bg in app/globals.css (the
+// popup surface), so the box blends into it while keeping >= 4.5:1 against
+// CHART_LEGEND_TEXT.dark.
+const SIGMA_HOVER_BG = Object.freeze({
+    light: '#ffffff',
+    dark: '#1a2330'
+});
+
+/**
+ * Dark-palette counterpart of sigma.js' default hover renderer (upstream
+ * drawHover, MIT): identical geometry — a rounded label box trailing a drop
+ * shadow, or a shadowed circle when the node has no label — but the box
+ * fills with the dark popup surface instead of the hardcoded white, so the
+ * light label color stays readable. Canvas-drawn, so CSS cannot reach it.
+ *
+ * @param {CanvasRenderingContext2D} context - Hover layer canvas context.
+ * @param {object} data - Display data (x, y, size, label) of the hovered node.
+ * @param {object} settings - Resolved Sigma settings (labelColor, labelFont…).
+ * @returns {void}
+ */
+export function drawDarkHover(context, data, settings) {
+    const size = settings.labelSize;
+    const font = settings.labelFont;
+    const weight = settings.labelWeight;
+
+    context.font = `${weight} ${size}px ${font}`;
+
+    // Label box background in the dark surface color.
+    context.fillStyle = SIGMA_HOVER_BG.dark;
+    context.shadowOffsetX = 0;
+    context.shadowOffsetY = 0;
+    context.shadowBlur = 8;
+    context.shadowColor = '#000';
+
+    const PADDING = 2;
+
+    if (typeof data.label === 'string') {
+        const textWidth = context.measureText(data.label).width;
+        const boxWidth = Math.round(textWidth + 5);
+        const boxHeight = Math.round(size + 2 * PADDING);
+        const radius = Math.max(data.size, size / 2) + PADDING;
+
+        const angleRadian = Math.asin(boxHeight / 2 / radius);
+        const xDeltaCoord = Math.sqrt(Math.abs(Math.pow(radius, 2) - Math.pow(boxHeight / 2, 2)));
+
+        context.beginPath();
+        context.moveTo(data.x + xDeltaCoord, data.y + boxHeight / 2);
+        context.lineTo(data.x + radius + boxWidth, data.y + boxHeight / 2);
+        context.lineTo(data.x + radius + boxWidth, data.y - boxHeight / 2);
+        context.lineTo(data.x + xDeltaCoord, data.y - boxHeight / 2);
+        context.arc(data.x, data.y, radius, angleRadian, -angleRadian);
+        context.closePath();
+        context.fill();
+    } else {
+        context.beginPath();
+        context.arc(data.x, data.y, data.size + PADDING, 0, Math.PI * 2);
+        context.closePath();
+        context.fill();
+    }
+
+    context.shadowOffsetX = 0;
+    context.shadowOffsetY = 0;
+    context.shadowBlur = 0;
+
+    // Label text in the same themed color sigma's default label renderer
+    // uses (settings.labelColor.color), mirroring upstream drawLabel.
+    if (data.label) {
+        context.fillStyle = settings.labelColor.color;
+        context.fillText(data.label, data.x + data.size + 3, data.y + size / 3);
+    }
+}
+
+/**
+ * Sigma settings for a 2D popup graph under the active palette: undefined in
+ * light mode (sigma defaults — #000 labels on the #FFF hover box — stay
+ * untouched), and in dark mode the chart legend text color for node labels
+ * plus the dark hover renderer above. Popups are rebuilt on every open, so
+ * resolving at open time suffices; the theme radios sit under the modal
+ * overlay while a popup is open.
+ *
+ * @returns {object|undefined} Partial Sigma settings, or undefined for sigma
+ *   to keep its defaults.
+ */
+export function sigmaPopupSettings() {
+    if (activeChartTheme() !== 'dark') return undefined;
+    return {
+        labelColor: {color: CHART_LEGEND_TEXT.dark},
+        hoverRenderer: drawDarkHover
+    };
+}
+
 export function showPopup(popupId, containerName, dot) {
     const overlay = document.getElementById('overlay');
     const popup = document.getElementById(popupId);
@@ -123,7 +216,7 @@ export function showPopup(popupId, containerName, dot) {
     popup.style.display = 'block';
     if (window.graphlibDot && window.graphology && window.Sigma) {
         const graph = buildGraphologyGraph(dot);
-        new window.Sigma(graph, document.getElementById(containerName));
+        new window.Sigma(graph, document.getElementById(containerName), sigmaPopupSettings());
     }
 }
 
 
@@ -1038,6 +1038,137 @@ describe('popup helpers', () => {
   });
 });
 
+describe('showPopup sigma theme settings', () => {
+  beforeEach(() => {
+    document.body.innerHTML = `
+      <div class="overlay" id="overlay" style="display:none"></div>
+      <div class="popup" id="popup-classGraph" style="display:none">
+        <div id="graph-container-classGraph"></div>
+      </div>`;
+  });
+
+  afterEach(() => {
+    delete window.graphlibDot;
+    delete window.graphology;
+    delete window.Sigma;
+    delete window.matchMedia;
+  });
+
+  // Installs the theme radios (which replace document.body.innerHTML) and
+  // re-appends the popup DOM they wiped, so showPopup finds its overlay.
+  function installRadiosAndPopup(mode, osPrefersDark) {
+    installThemeRadios(mode, osPrefersDark);
+    document.body.insertAdjacentHTML('beforeend', `
+      <div class="overlay" id="overlay" style="display:none"></div>
+      <div class="popup" id="popup-classGraph" style="display:none">
+        <div id="graph-container-classGraph"></div>
+      </div>`);
+  }
+
+  // Minimal widget stubs so showPopup reaches the Sigma constructor: an
+  // empty graphlib-dot graph (the sugiyama layout then no-ops) and a Sigma
+  // "constructor" recording its arguments.
+  function stubSigmaStack() {
+    const constructed = [];
+    window.graphlibDot = { read: () => ({ nodes: () => [], edges: () => [] }) };
+    window.graphology = {
+      Graph: class {
+        addNode() {}
+        addEdge() {}
+        nodes() { return []; }
+        forEachNeighbor() {}
+        setNodeAttribute() {}
+      }
+    };
+    window.Sigma = function (graph, container, settings) {
+      constructed.push({ graph, container, settings });
+    };
+    return constructed;
+  }
+
+  it('keeps the sigma defaults in light mode', () => {
+    installRadiosAndPopup('light');
+    const constructed = stubSigmaStack();
+    showPopup('popup-classGraph', 'graph-container-classGraph', 'strict digraph G {}');
+    expect(constructed.length).toBe(1);
+    expect(constructed[0].settings).toBeUndefined();
+  });
+
+  it('passes the dark legend text color and a hover renderer in dark mode', () => {
+    installRadiosAndPopup('dark');
+    const constructed = stubSigmaStack();
+    showPopup('popup-classGraph', 'graph-container-classGraph', 'strict digraph G {}');
+    const settings = constructed[0].settings;
+    expect(settings.labelColor.color).toBe('#9fb0c0');
+    expect(typeof settings.hoverRenderer).toBe('function');
+  });
+
+  it('resolves the dark settings for the system radio when the OS prefers dark', () => {
+    installRadiosAndPopup('system', true);
+    const constructed = stubSigmaStack();
+    showPopup('popup-classGraph', 'graph-container-classGraph', 'strict digraph G {}');
+    expect(constructed[0].settings.labelColor.color).toBe('#9fb0c0');
+  });
+
+  it('draws the hover box in the dark popup surface with light label text', () => {
+    installRadiosAndPopup('dark');
+    const constructed = stubSigmaStack();
+    showPopup('popup-classGraph', 'graph-container-classGraph', 'strict digraph G {}');
+    // Representative resolved settings, as sigma's resolveSettings merges the
+    // popup partial over its DEFAULT_SETTINGS (labelSize 14, Arial, normal).
+    const resolved = {
+      labelSize: 14,
+      labelFont: 'Arial',
+      labelWeight: 'normal',
+      ...constructed[0].settings
+    };
+
+    const fills = [];
+    const drawn = [];
+    const coords = [];
+    const context = {
+      font: '',
+      shadowOffsetX: 0, shadowOffsetY: 0, shadowBlur: 0, shadowColor: '',
+      measureText: () => ({ width: 42 }),
+      beginPath() {}, closePath() {},
+      moveTo: (...args) => coords.push(...args),
+      lineTo: (...args) => coords.push(...args),
+      arc: (...args) => coords.push(...args),
+      fill() { drawn.push({ op: 'fill', color: fills[fills.length - 1] }); },
+      fillText(text, ...args) {
+        drawn.push({ op: 'text', text, color: fills[fills.length - 1] });
+        coords.push(...args);
+      },
+      set fillStyle(value) { fills.push(value); },
+      get fillStyle() { return fills[fills.length - 1]; }
+    };
+    resolved.hoverRenderer(context, { x: 10, y: 20, size: 5, label: 'Foo' }, resolved);
+    // The label-box geometry must be finite everywhere (NaN coordinates would
+    // silently skip painting the box and the label).
+    expect(coords.length).toBeGreaterThan(0);
+    expect(coords.every(Number.isFinite)).toBe(true);
+    // The label font comes from the resolved label settings.
+    expect(context.font).toBe('normal 14px Arial');
+    // The hovered node's label box fills with the dark --card-bg surface…
+    expect(drawn.some(d => d.op === 'fill' && d.color === '#1a2330')).toBe(true);
+    // …and its text uses the dark chart legend color (>= 4.5:1 on it).
+    expect(drawn.some(d => d.op === 'text' && d.text === 'Foo' && d.color === '#9fb0c0')).toBe(true);
+  });
+
+  it('keeps the dark sigma label readable on the dark hover box (>= 4.5:1)', () => {
+    // #1a2330 is the dark palette's --card-bg (the popup surface) in
+    // app/globals.css; #9fb0c0 is CHART_LEGEND_TEXT.dark.
+    expect(contrastAgainst('#9fb0c0', [0x1a, 0x23, 0x30])).toBeGreaterThanOrEqual(4.5);
+  });
+
+  it('falls back to sigma defaults for the system radio when the OS prefers light', () => {
+    installRadiosAndPopup('system', false);
+    const constructed = stubSigmaStack();
+    showPopup('popup-classGraph', 'graph-container-classGraph', 'strict digraph G {}');
+    expect(constructed[0].settings).toBeUndefined();
+  });
+});
+
 describe('bindPopupHandlers (template uses data attributes, no inline handlers)', () => {
   beforeEach(() => {
     document.body.innerHTML = `