[Fleet] Use theme service to display in Dark Mode when set on User Pr…

…ofile (elastic#161819)

Closes elastic#161135

## What does this PR do?
* Fixes the darkMode theme in fleet page.

## Issue References
* elastic#161135

## Video/Screenshot Demo 
##### Before

![250689182-c9c4fe55-c007-424a-aedc-b863f1fd0c88-2](https://github.com/elastic/kibana/assets/1501599/71dd9889-3e2d-4b5b-8825-5f7f18206903)

![250689299-34149e80-40b8-493d-a8fc-3079c8cb199c](https://github.com/elastic/kibana/assets/1501599/fdd1b8af-6a5f-4e6c-892e-73baf58d1aab)

###### FIX:

![250690628-2774dc86-487e-489e-b0a2-a1c06539172d](https://github.com/elastic/kibana/assets/1501599/4525bbfa-7771-4611-a099-751c7be54ec6)

![250691578-74d860fa-1db5-47c4-a3b7-425e86720e06](https://github.com/elastic/kibana/assets/1501599/0b03267a-dfb7-48dd-adc3-e9fc3ad5013b)

---
This code was written and reviewed by GitStart Community. Growing future
engineers, one PR at a time.

---------

Co-authored-by: KlingerMatheus <klinger.matheus@gitstart.dev>
Co-authored-by: gitstart_bot <gitstart_bot@users.noreply.github.com>
Co-authored-by: Klinger Matheus <50892465+KlingerMatheus@users.noreply.github.com>
Co-authored-by: Julia Bardi <90178898+juliaElastic@users.noreply.github.com>
Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

x-pack/plugins/fleet/public/applications/fleet/app.tsx

@@ -255,7 +255,8 @@ export const FleetAppContext: React.FC<{
     theme$,
     fleetStatus,
   }) => {
-    const isDarkMode = useObservable<boolean>(startServices.uiSettings.get$('theme:darkMode'));
+    const darkModeObservable = useObservable(startServices.theme.theme$);
+    const isDarkMode = darkModeObservable && darkModeObservable.darkMode;
 
     return (
       <RedirectAppLinks application={startServices.application}>

x-pack/plugins/fleet/public/applications/fleet/sections/agent_policy/create_package_policy_page/components/steps/step_select_agent_policy.test.tsx

@@ -59,7 +59,7 @@ describe('step select agent policy', () => {
   });
 
   test('should not select agent policy by default if multiple exists', async () => {
-    useGetAgentPoliciesMock.mockReturnValueOnce({
+    useGetAgentPoliciesMock.mockReturnValue({
       data: {
         items: [
           { id: 'policy-1', name: 'Policy 1' },