message: feat(GUI): use gtk3 dark theme mode
  We enable the `darkTheme` mode for applications (mainly Linux)
  that suits Etcher's dark theme better, making the window title bar dark.
  Change-Type: patch
  Changelog-Entry: Use darkTheme mode.
diff: @@ -48,6 +48,7 @@ electron.app.on('ready', () => {
  autoHideMenuBar: true,
  titleBarStyle: 'hidden-inset',
  icon: path.join(__dirname, '..', '..', 'assets', 'icon.png'),
  + darkTheme: true,
  webPreferences: {
  backgroundThrottling: false
  }
  