A PowerShell script setting light mode during the day and dark mode after 18:30.

windows-auto-darkmode.ps1

# Set your triggers to 06:00, log in, and 18:30. 

# Set this to false if you like light Windows UI.
# Personally, I'm conflicted so I change it back and forth every few months.
$SYSTEM_ALWAYS_DARK = $TRUE

$lightTheme = @{ AppsUseLightTheme = 1; SystemUsesLightTheme = 1 }
$darkTheme = @{ AppsUseLightTheme = 0; SystemUsesLightTheme = 0 }

# We set light mode between 06:00 and 18:30.
$time = Get-Date -Format 'HH:mm'
$isDay = $time -ge '06:00' -and $time -le '18:30'

if ($isDay) {
  $settings = $lightTheme
} else {
  $settings = $darkTheme
}

if ($SYSTEM_ALWAYS_DARK) {
  $settings.SystemUsesLightTheme = 0
}

foreach ($key in $settings.Keys) {
  $value = $settings[$key]

  Set-ItemProperty -Path "HKCU:\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize" -Name $key -Value $value
}