Captcha Settings
From: Using Back-End
Every extension and theme can use Captcha system from core settings. You can switch between Captcha systems for your entire website just by changing the settings.
To manage Captcha settings, go to the Captcha tab in Settings/Security. You can select Maincaptcha or Google reCaptcha as default Captcha system.
Maincaptcha
Maincaptcha is very simple Captcha system in core.
Theme: Captcha with Dark or Light background.
Picture: Reset button picture. The default reset button is Reset text.
Google reCaptcha
Version: You can use version 2 or 3.
Site Key: Google reCaptcha Site Key.
Secret Key: Google reCaptcha Secret Key.
Theme: Captcha with Dark or Light background.
Size: Captcha with Normal or Compact size. Only works on version 2.