WinUI 3 branch - Theme Mode is not working in the settings

[LeftTwixWand]

Describe the bug I'm testing the winui3 branch - Theme mode is not working in the settings. But in the same time, some controls samples can change a theme of controls. Just like TeachinTip sample:

Screenshots

Version Info

• [ ] App downloaded from Store
  • If so, which version? (Found on the Settings page of the app.)
• [x] App built locally from source
  • Branch: winui3

Windows 10 version:

• [ ] Insider Build (xxxxx)
• [x] May 2020 Update (19041)
• [ ] November 2019 Update (18363)
• [ ] May 2019 Update (18362)
• [ ] October 2018 Update (17763)
• [ ] April 2018 Update (17134)
• [ ] Fall Creators Update (16299)
• [ ] Creators Update (15063)
• [ ] Anniversary Update (14393)

Device form factor:

• [x] Desktop
• [ ] Mobile
• [ ] Xbox
• [ ] Surface Hub
• [ ] IoT