Use this code to change the styles of the print certificate button in LearnDash

learndash-print-certificate-button-style.css

/**
 * This will change the styles of the 'Print Your Certificate' button only.
 *
 * These styles should be copied & pasted to a child theme or the "Additional CSS"
 * area of the WordPress Customizer.
 *
 */

/* Update the colors to match your theme, and feel free to add additional
 * styles as well (border radius, text transform, etc.)
 */

/* normal button state */
.learndash .learndash_course_certificate .btn-blue,
.learndash .learndash_course_certificate .btn-blue:visited {
	background: #000;
	color: #fff;
}
/* hover state */
.learndash .learndash_course_certificate .btn-blue:hover,
.learndash .learndash_course_certificate .btn-blue:active,
.learndash .learndash_course_certificate .btn-blue:focus {
	background: #000;
	color: #fff;
}