How can I modify the styles/CSS of the checkbox?
The checkbox on the login page that reads "I'm a human" has its own styles applied to it to fit in with the standard WordPress login page styles.
But not everyone uses this standard page styling, so some of you may want to adjust it to suit.
Since the checkbox is dynamically generated some of the properties are also dynamic, but it's still possible to adjust the styles. To do this, you need to add your custom styles to your theme (how you do this depends on your theme - consult your developer or theme provider).
Add the following to your CSS/custom styles to remove all borders, create a white background and make the text inside red.
border: 0 none;
You can change your styles to however you need it to look... this is just an example of what you can do and how to do it.