diff --git a/ui/src/components/MarkdownBody.tsx b/ui/src/components/MarkdownBody.tsx
index 81405c4b61f..a26cafac79e 100644
--- a/ui/src/components/MarkdownBody.tsx
+++ b/ui/src/components/MarkdownBody.tsx
@@ -231,7 +231,7 @@ const codeBlockActionsStyle: React.CSSProperties = {
   gap: "0.25rem",
 };
 
-const codeBlockActionStyle: React.CSSProperties = {
+export const codeBlockActionStyle: React.CSSProperties = {
   position: "static",
   opacity: 1,
   display: "inline-flex",
diff --git a/ui/src/components/MarkdownCodeBlockStyles.test.ts b/ui/src/components/MarkdownCodeBlockStyles.test.ts
new file mode 100644
index 00000000000..f269e90bf86
--- /dev/null
+++ b/ui/src/components/MarkdownCodeBlockStyles.test.ts
@@ -0,0 +1,70 @@
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { describe, expect, it } from "vitest";
+
+import { codeBlockActionStyle } from "./MarkdownBody";
+
+const stylesheet = readFileSync(fileURLToPath(new URL("../index.css", import.meta.url)), "utf8");
+
+function cssBlock(selector: string): string {
+  const start = stylesheet.indexOf(`${selector} {`);
+  expect(start, `Missing CSS selector: ${selector}`).toBeGreaterThanOrEqual(0);
+
+  const bodyStart = stylesheet.indexOf("{", start);
+  const bodyEnd = stylesheet.indexOf("\n}", bodyStart);
+  expect(bodyStart, `Missing CSS block start: ${selector}`).toBeGreaterThanOrEqual(0);
+  expect(bodyEnd, `Missing CSS block end: ${selector}`).toBeGreaterThan(bodyStart);
+
+  return stylesheet.slice(bodyStart + 1, bodyEnd);
+}
+
+/* The rendered code block used to be pinned to the Catppuccin literals
+   #1e1e2e / #cdd6f4, in the normal AND the prose-invert variables. A code
+   block therefore stayed dark in light mode. These tests fail if any of
+   those surfaces is pinned to a literal again, rather than riding a token
+   that carries a `.dark` override. */
+
+describe("markdown code block styles", () => {
+  it("rides theme tokens for the rendered code block surface", () => {
+    const block = cssBlock(".paperclip-markdown pre");
+
+    expect(block).toContain("background-color: var(--muted)");
+    expect(block).toContain("color: var(--foreground)");
+    expect(block).toContain("border: 1px solid var(--border)");
+    expect(block).toContain("border-radius: var(--radius-lg)");
+  });
+
+  it("rides theme tokens for the editor content code block surface", () => {
+    const block = cssBlock(".paperclip-mdxeditor-content pre");
+
+    expect(block).toContain("background: var(--muted)");
+    expect(block).toContain("color: var(--foreground)");
+    expect(block).toContain("border: 1px solid var(--border)");
+    expect(block).toContain("border-radius: var(--radius-lg)");
+  });
+
+  it("points the normal and inverted prose variables at the same tokens", () => {
+    const block = cssBlock(".paperclip-markdown");
+
+    expect(block).toContain("--tw-prose-pre-bg: var(--muted)");
+    expect(block).toContain("--tw-prose-pre-code: var(--foreground)");
+    expect(block).toContain("--tw-prose-invert-pre-bg: var(--muted)");
+    expect(block).toContain("--tw-prose-invert-pre-code: var(--foreground)");
+  });
+
+  it("keeps every themed code surface free of hardcoded colors", () => {
+    for (const selector of [".paperclip-markdown pre", ".paperclip-mdxeditor-content pre", ".paperclip-markdown"]) {
+      // Any hex literal, or an rgb()/hsl() literal, re-pins the surface to one mode.
+      expect(cssBlock(selector), `${selector} must not hardcode a color`).not.toMatch(
+        /#[0-9a-f]{3,8}\b|\brgba?\(|\bhsla?\(/i,
+      );
+    }
+  });
+
+  it("separates the copy and wrap controls from the code block surface", () => {
+    // The controls sit on top of the block. --muted is now the block's own
+    // fill, so the controls must not use it or they disappear against it.
+    expect(codeBlockActionStyle.backgroundColor).toBe("var(--background)");
+    expect(codeBlockActionStyle.backgroundColor).not.toContain("--muted");
+  });
+});