Add comprehensive test suite, fix minor issues, rename function and file names to match approved verbs (#708)

Scripts/GUI/Get-SystemUsesDarkMode.ps1

@@ -0,0 +1,22 @@
+<#
+    .SYNOPSIS
+        Returns whether Windows apps are configured to use dark mode.
+
+    .OUTPUTS
+        System.Boolean. $false when the personalization setting cannot be read.
+#>
+function Get-SystemUsesDarkMode {
+    try {
+        $personalizeKey = Get-ItemProperty -Path 'HKCU:\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize'
+
+        if ($null -eq $personalizeKey) {
+            Write-Host "WARNING: Unable to retrieve personalization settings." -ForegroundColor Yellow
+            return $false
+        }
+
+        return $personalizeKey.AppsUseLightTheme -eq 0
+    }
+    catch {
+        return $false
+    }
+}