I changed all the portal colors to a dark theme and found that on pages like the registration page, the captcha image had a different border color around it than the rest of the borders present on the page.  So whatever method is being used to generate the border colors uses a different technique for the captcha.

To make the border colors match I had to add a custom CSS rule:

.captcha-form-element .captcha-image>img { border: solid 1px #777777 !important; }