diff --git a/apps/desktop/src/preview/Manager.test.ts b/apps/desktop/src/preview/Manager.test.ts
index d3f6971b820b537e826806a0c28b9c4e50711b2b..95197d8916a05e9a29dcc58f01391c856ef245e3 100644
--- a/apps/desktop/src/preview/Manager.test.ts
+++ b/apps/desktop/src/preview/Manager.test.ts
@@ -1601,7 +1601,7 @@ describe("PreviewManager", () => {
     ),
   );
 
-  effectIt.effect("emulates prefers-color-scheme and re-applies it across webview swaps", () =>
+  effectIt.effect("re-applies opaque base and color scheme across webview swaps", () =>
     withManager((manager) =>
       Effect.gen(function* () {
         const makeWebContents = (id: number) => {
@@ -1650,6 +1650,16 @@ describe("PreviewManager", () => {
         yield* manager.registerWebview("tab_scheme", 42);
         yield* Effect.yieldNow;
 
+        // Guests start with a transparent base; dark-scheme pages need an opaque
+        // one or Chromium skips their dark canvas.
+        const opaqueBase = {
+          color: { r: 255, g: 255, b: 255, a: 1 },
+        };
+        expect(first.sendCommand).toHaveBeenCalledWith(
+          "Emulation.setDefaultBackgroundColorOverride",
+          opaqueBase,
+        );
+
         yield* manager.setColorScheme("tab_scheme", "dark");
 
         expect(first.sendCommand).toHaveBeenCalledWith("Emulation.setEmulatedMedia", {
@@ -1662,6 +1672,10 @@ describe("PreviewManager", () => {
         yield* manager.registerWebview("tab_scheme", 43);
         yield* Effect.yieldNow;
 
+        expect(replacement.sendCommand).toHaveBeenCalledWith(
+          "Emulation.setDefaultBackgroundColorOverride",
+          opaqueBase,
+        );
         expect(replacement.sendCommand).toHaveBeenCalledWith("Emulation.setEmulatedMedia", {
           features: [{ name: "prefers-color-scheme", value: "dark" }],
         });
diff --git a/apps/desktop/src/preview/Manager.ts b/apps/desktop/src/preview/Manager.ts
index cc1a8f524dc8a507e238b6eb9499a4e1b25d7dfc..4195bdffff6a6672ff43eed9edae8799036f6cab 100644
--- a/apps/desktop/src/preview/Manager.ts
+++ b/apps/desktop/src/preview/Manager.ts
@@ -1418,6 +1418,19 @@ const makeNativeOperations = Effect.fn("PreviewManager.makeOperations")(function
                 ),
               { concurrency: "unbounded", discard: true },
             );
+            // Electron gives `<webview>` guests a transparent base background, and
+            // Chromium only paints a dark canvas for dark color-scheme pages over an
+            // opaque base. Without this, dark-scheme pages with no background of
+            // their own (text/plain, e.g. .md files) render white text on white.
+            // White matches the webview's existing white backing, so light pages look
+            // the same; Chromium still swaps in its dark canvas for dark-scheme pages.
+            yield* attemptPromise(
+              { operation: "initializeDebugger.defaultBackground", webContentsId: wc.id },
+              () =>
+                wcDebugger.sendCommand("Emulation.setDefaultBackgroundColorOverride", {
+                  color: { r: 255, g: 255, b: 255, a: 1 },
+                }),
+            );
             return [
               control,
               replaceMap(sessions, (copy) => {