modify style css on Bp SC

The entire security question section is wrapped in a div with the class security-question-section. So you can use CSS selectors like these:

Security Question heading
.security-question-section h4 {}

Answer input field
.security-question-section input[type="number] {}
.security-question-section input[name="bp_security_check"] {}

Sum text
.security-question-section label {}
.security-question-section label[for="bp-security-check"] {}

To override built-in styles, use the !important declaration.