Ok, I've come up with a hack that works but is far from ideal.
Open core/components/quip/model/recaptcha/recaptcha.class.php
Replace line 128:
return '<script type="text/javascript">var RecaptchaOptions = '.$this->modx->toJSON($opt).';</script><script type="text/javascript" src="'. $server . 'challenge?k=' . $this->config[reCaptcha::OPT_PUBLIC_KEY] . $errorpart . '"></script>
With:
return '<script type="text/javascript">var RecaptchaOptions = {"theme":"red","lang":"en"};</script><script type="text/javascript" src="'. $server . 'challenge?k=' . $this->config[reCaptcha::OPT_PUBLIC_KEY] . $errorpart . '"></script>
(change "red" to the theme you would like)