Set Windows 11 to use Dark Mode. Particularly useful as part of a logon script for Windows Sandbox.

Set-DarkTheme.ps1

$strRegJson = @"
[
    {
    "RegPath": "HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\CurrentVersion\\Themes",
    "Name": "CurrentTheme",
    "Type": "STRING",
    "Value": "C:\\Windows\\resources\\Themes\\dark.theme"
    },
    {
    "RegPath": "HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\HighContrast",
    "Name": "Pre-High Contrast Scheme",
    "Type": "STRING",
    "Value": "C:\\Windows\\resources\\Themes\\dark.theme"

    },
    {
    "RegPath": "HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
    "Name": "AppsUseLightTheme",
    "Type": "DWORD",
    "Value": 0

    },
    {
    "RegPath": "HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
    "Name": "SystemUsesLightTheme",
    "Type": "DWORD",
    "Value": 0

    },
    {
    "RegPath": "HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\CurrentVersion\\Themes",
    "Name": "ThemeMRU",
    "Type": "STRING",
    "Value": "C:\\Windows\\resources\\Themes\\dark.theme;C:\\Windows\\resources\\Themes\\aero.theme;"

    },
    {
    "RegPath": "HKEY_CURRENT_USER\\Control Panel\\Desktop",
    "Name": "WallPaper",
    "Type": "STRING",
    "Value": "C:\\Windows\\web\\wallpaper\\Windows\\img19.jpg"
    }
]
"@

Add-Type @"
using System;
using System.Runtime.InteropServices;

public class User32 {
    [DllImport("user32.dll", SetLastError = true)]
    public static extern bool SystemParametersInfo(uint action, uint param, IntPtr vparam, uint init);
}
"@

$objRegJson = $strRegJson | ConvertFrom-Json

if ($objRegJson) {

    taskkill /f /im explorer.exe 2>&1> $null

    $objRegJson | ForEach-Object {

        if ($null -notin ($_.RegPath, $_.Name, $_.Type, $_.Value)) {

            $regPath = switch -Regex ($_.RegPath) {
                '^(HKEY_CURRENT_USER)' {$_ -replace $Matches[1], 'HKCU:'}
                #'^(HKEY_LOCAL_MACHINE)' {$_ -replace $Matches[1], 'HKLM:'}
                Default {continue}
            }

            try {
                Set-ItemProperty -Path $regPath -Name $_.Name -Type $_.Type -Value $_.Value
            }
            catch {
                throw 'An error occured whilst adding items to the registry.'
            }
        }
    }

    $SPI_SETDESKWALLPAPER = 0x0014
    $SPIF_UPDATEINIFILE = 0x01
    $SPIF_SENDCHANGE = 0x02

    [User32]::SystemParametersInfo($SPI_SETDESKWALLPAPER, 0, [IntPtr]::Zero, $SPIF_UPDATEINIFILE -bor $SPIF_SENDCHANGE) | Out-Null

    Start-Process explorer.exe
}