Add dark theme and secret key creation script

scripts/generate-secret-key.ps1

@@ -0,0 +1,9 @@
+# Prints a cryptographically random SECRET_KEY value; does not modify .env.
+$bytes = [byte[]]::new(32)
+$rng = [System.Security.Cryptography.RNGCryptoServiceProvider]::new()
+try {
+    $rng.GetBytes($bytes)
+} finally {
+    $rng.Dispose()
+}
+($bytes | ForEach-Object { $_.ToString("x2") }) -join ""