FN-6826: elevate the theme dropdown when open

Ensure the Command Center theme selector renders above neighboring views while preserving mobile in-flow behavior.

- Add an open state class to the theme dropdown root for local stacking control.
- Raise the open dropdown and popover above Command Center sibling cards without exceeding app chrome layers.
- Cover desktop elevation and mobile static popover behavior in ThemeDropdown tests.

Files changed:
 .../dashboard/app/components/ThemeDropdown.css     | 15 +++++++-
 .../dashboard/app/components/ThemeDropdown.tsx     |  2 +-
 .../components/__tests__/ThemeDropdown.test.tsx    | 40 ++++++++++++++++++++++
 3 files changed, 55 insertions(+), 2 deletions(-)

Fusion-Task-Id: FN-6826

Fusion-Task-Lineage: ff43c06e-f3a7-4b4c-8e29-46deb66e9145

packages/dashboard/app/components/ThemeDropdown.css

@@ -6,6 +6,14 @@
   min-inline-size: 0;
 }
 
+/*
+FNXC:Theme 2026-06-20-00:00:
+FN-6826 requires the Command Center theme dropdown to paint on top of all sibling Command Center cards/views only while open. Use a local dropdown-tier z-index above DateRangePicker's 20 and below app chrome/mobile nav tiers, then reset it in the mobile in-flow branch so closed or static content never floats above unrelated chrome.
+*/
+.theme-dropdown.open {
+  z-index: 40;
+}
+
 .theme-dropdown-trigger {
   justify-content: space-between;
   gap: var(--space-sm);
@@ -43,7 +51,7 @@
 
 .theme-dropdown-popover {
   position: absolute;
-  z-index: 10;
+  z-index: 40;
   inset-block-start: calc(100% + var(--space-xs));
   inset-inline: 0;
   padding: var(--space-sm);
@@ -85,8 +93,13 @@
 }
 
 @media (max-width: 768px) {
+  .theme-dropdown.open {
+    z-index: auto;
+  }
+
   .theme-dropdown-popover {
     position: static;
+    z-index: auto;
     margin-block-start: var(--space-xs);
   }
 

packages/dashboard/app/components/ThemeDropdown.tsx

@@ -105,7 +105,7 @@ export function ThemeDropdown({ colorTheme, onColorThemeChange, themeMode, onThe
   };
 
   return (
-    <div className="theme-dropdown" ref={rootRef}>
+    <div className={`theme-dropdown${open ? " open" : ""}`} ref={rootRef}>
       <button
         type="button"
         className="theme-dropdown-trigger btn"

packages/dashboard/app/components/__tests__/ThemeDropdown.test.tsx

@@ -1,3 +1,4 @@
+import { readFileSync } from "node:fs";
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import { fireEvent, render, screen, within } from "@testing-library/react";
 import { COLOR_THEMES } from "../themeOptions";
@@ -73,4 +74,43 @@ describe("ThemeDropdown", () => {
     fireEvent.click(screen.getByRole("button", { name: /light/i }));
     expect(onThemeModeChange).toHaveBeenCalledWith("light");
   });
+
+  it.each([
+    ["without mode controls", undefined, undefined],
+    ["with mode controls", "dark" as const, vi.fn()],
+  ])("elevates the open popover above Command Center sibling cards %s", (_label, themeMode, onThemeModeChange) => {
+    render(
+      <ThemeDropdown
+        colorTheme="default"
+        themeMode={themeMode}
+        onColorThemeChange={vi.fn()}
+        onThemeModeChange={onThemeModeChange}
+      />,
+    );
+
+    const trigger = screen.getByRole("button", { name: /default/i });
+    const root = trigger.closest(".theme-dropdown");
+    expect(root).toBeTruthy();
+    expect(root?.classList.contains("open")).toBe(false);
+    expect(getComputedStyle(root!).zIndex).not.toBe("40");
+
+    fireEvent.click(trigger);
+
+    const popover = document.querySelector<HTMLElement>(".theme-dropdown-popover");
+    expect(trigger.getAttribute("aria-expanded")).toBe("true");
+    expect(root?.classList.contains("open")).toBe(true);
+    expect(getComputedStyle(root!).position).toBe("relative");
+    expect(getComputedStyle(root!).zIndex).toBe("40");
+    expect(popover).toBeTruthy();
+    expect(getComputedStyle(popover!).position).toBe("absolute");
+    expect(getComputedStyle(popover!).zIndex).toBe("40");
+  });
+
+  it("preserves the mobile static in-flow popover branch without dropdown elevation", () => {
+    const css = readFileSync("app/components/ThemeDropdown.css", "utf8");
+
+    expect(css).toMatch(
+      /@media \(max-width: 768px\) \{[\s\S]*?\.theme-dropdown\.open \{[\s\S]*?z-index: auto;[\s\S]*?\.theme-dropdown-popover \{[\s\S]*?position: static;[\s\S]*?z-index: auto;/,
+    );
+  });
 });