diff --git a/addons/xterm-addon-serialize/src/SerializeAddon.test.ts b/addons/xterm-addon-serialize/src/SerializeAddon.test.ts
index 4c5dd645..75148ab9 100644
--- a/addons/xterm-addon-serialize/src/SerializeAddon.test.ts
+++ b/addons/xterm-addon-serialize/src/SerializeAddon.test.ts
@@ -74,7 +74,7 @@ describe('xterm-addon-serialize', () => {
     terminal = new Terminal({ cols: 10, rows: 2, allowProposedApi: true });
     terminal.loadAddon(serializeAddon);
 
-    (terminal as any)._core._themeService = new ThemeService(new OptionsService({}));
+    (terminal as any)._core._themeService = new ThemeService((terminal as any)._core.optionsService);
     (terminal as any)._core._selectionService = new TestSelectionService((terminal as any)._core._bufferService);
   });
 
@@ -205,5 +205,25 @@ describe('xterm-addon-serialize', () => {
       });
       assert.equal((output.match(/color: #ffffff; background-color: #000000; font-family: courier-new, courier, monospace; font-size: 15px;/g) || []).length, 1, output);
     });
+
+    it('cells with custom color styling', async () => {
+      terminal.options.theme.black = '#ffa500';
+      terminal.options.theme = { ... terminal.options.theme };
+
+      await writeP(terminal, ' ' + sgr('38;5;0') + 'terminal' + sgr('39') + ' ');
+
+      const output = serializeAddon.serializeAsHTML();
+      assert.equal((output.match(/<span style='color: #ffa500;'>terminal<\/span>/g) || []).length, 1, output);
+    });
+
+    it('cells with color styling - xterm headless', async () => {
+      // a headless terminal doesn't have a themeservice
+      (terminal as any)._core._themeService = undefined;
+
+      await writeP(terminal, ' ' + sgr('38;5;46') + 'terminal' + sgr('39') + ' ');
+
+      const output = serializeAddon.serializeAsHTML();
+      assert.equal((output.match(/<span style='color: #00ff00;'>terminal<\/span>/g) || []).length, 1, output);
+    });
   });
 });
diff --git a/addons/xterm-addon-serialize/src/SerializeAddon.ts b/addons/xterm-addon-serialize/src/SerializeAddon.ts
index b83ff160..50c200a3 100644
--- a/addons/xterm-addon-serialize/src/SerializeAddon.ts
+++ b/addons/xterm-addon-serialize/src/SerializeAddon.ts
@@ -7,7 +7,8 @@
 
 import { Terminal, ITerminalAddon, IBuffer, IBufferCell, IBufferRange } from 'xterm';
 import { IColorSet } from 'browser/Types';
-import { IAttributeData } from 'common/Types';
+import { IAttributeData, IColor } from 'common/Types';
+import { DEFAULT_ANSI_COLORS } from 'browser/services/ThemeService';
 
 function constrain(value: number, low: number, high: number): number {
   return Math.max(low, Math.min(value, high));
@@ -534,7 +535,7 @@ export class HTMLSerializeHandler extends BaseSerializeHandler {
 
   private _htmlContent = '';
 
-  private _colors: IColorSet;
+  private _ansiColors: Readonly<IColor[]>;
 
   constructor(
     buffer: IBuffer,
@@ -543,8 +544,13 @@ export class HTMLSerializeHandler extends BaseSerializeHandler {
   ) {
     super(buffer);
 
-    // https://github.com/xtermjs/xterm.js/issues/3601
-    this._colors = (_terminal as any)._core._themeService.colors;
+    // For xterm headless: fallback to ansi colors
+    if ((_terminal as any)._core._themeService) {
+      this._ansiColors = (_terminal as any)._core._themeService.colors.ansi;
+    }
+    else {
+      this._ansiColors = DEFAULT_ANSI_COLORS;
+    }
   }
 
   private _padStart(target: string, targetLength: number, padString: string): string {
@@ -600,7 +606,7 @@ export class HTMLSerializeHandler extends BaseSerializeHandler {
       return rgb.map(x => this._padStart(x.toString(16), 2, '0')).join('');
     }
     if (isFg ? cell.isFgPalette() : cell.isBgPalette()) {
-      return this._colors.ansi[color].css;
+      return this._ansiColors[color].css;
     }
     return undefined;
   }