The following information is taken in part from here
Customizing the Look and Feel of reCAPTCHA
To make the reCAPTCHA widget display a different theme, you first need to add the following code in your main HTML page anywhere before the <form> element where reCAPTCHA appears. WHMCS would recommend placing this in the header.tpl file of your template just before the </head> tag.
The 'theme_name' can be one of four pre-defined options
If you do not wish to use any of the default themes, you can create your own to match your site design. You can see how to do this @ Custom Theme. To use a custom theme, the theme variable must be set to 'custom'.
If your language is not one supported by Google, you are able to create your own translations. More information on this at https://developers.google.com/recaptcha/old/docs/customization#custom-translations