[Windows] Title bar does not respect dark mode

[zlstringham]

Preflight Checklist

• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Issue Details

When setting nativeTheme.themeSource to 'dark' (or 'system' and setting the system theme to 'dark'), the title bar on Windows remains light. On macOS, the title bar accurately reflects the setting.

• Electron Version:
  • npm ./node_modules/.bin/electron --version: 6.9.0
  • package.json: "electron": "^8.2.5",
  • yarn.lock: 8.2.5
• Operating System:
  • Windows 10 (1903)

Expected Behavior

(This works as expected on macOS)

	nativeTheme.themeSource = 'system'	nativeTheme.themeSource = 'light'	nativeTheme.themeSource = 'dark'
Light Theme	Light Title Bar	Light Title Bar	Dark Title Bar
Dark Theme	Dark Title Bar	Light Title Bar	Dark Title Bar

Actual Behavior

	nativeTheme.themeSource = 'system'	nativeTheme.themeSource = 'light'	nativeTheme.themeSource = 'dark'
Light Theme	Light Title Bar	Light Title Bar	Light Title Bar
Dark Theme	Light Title Bar	Light Title Bar	Light Title Bar

To Reproduce

nativeTheme.themeSource = 'dark'

Screenshots

Command Prompt:

File Explorer:

Title Bar in Electron with nativeTheme.themeSource = 'dark'

[miniak]

This is only doable using undocumented APIs for win32 applications
https://stackoverflow.com/questions/53501268/win10-dark-theme-how-to-use-in-winapi
https://github.com/ysc3839/win32-darkmode

[ckerr]

I have a WIP for this and will push it to a public branch RSN. Hoping to have this ready by the end of the week.

[ckerr]

I have updated the patch based on review feedback.

In this week's Releases Workgroup meeting, I'll propose its inclusion. The point of contention in review is that a fair amount of private API is necessary to make this work.

[ckerr]

An agreement has been reached about how to handle use of nonpublic APIs -- this PR will be merged behind a compile-time feature flag.

[ckerr]

After a few more revision requests, the PR to honor nativeTheme.themeSource = 'dark' being called before the Window is created has finally been approved. The backport bot 'trop' can port it from master to 11; I'll backport it to 10 manually.

[ckerr]

This has been merged into Electron 10, 11, and master behind a disabled build flag. It can be enabled by changing enable_win_dark_mode_window_ui to true in buildflags/buildflags.gni.