If WCAG (Accessibility) is enabled, then there is no way to change its color/style using CSS. A radio button is a native element specific to each operating system and browser. Implement custom images or use a custom JavaScript library, which requires a full custom HTML survey, in cases where these elements should not be changed and a WCAG is required. This approach is not recommended as it unnecessarily increases complexity of the survey design process and increases the time taken to modify and publish surveys.

The following CSS works if WCAG is disabled for an account. Request this change from your Medallia expert:

.neb-content md-radio-group div.radio md-radio-button ._md-on {
    background-color: #c51162 !important;
}