source: twentyseventeen/3.6/inc/custom-header.php

<?php
/**
 * Custom header implementation
 *
 * @link https://codex.wordpress.org/Custom_Headers
 *
 * @package WordPress
 * @subpackage Twenty_Seventeen
 * @since Twenty Seventeen 1.0
 */

/**
 * Set up the WordPress core custom header feature.
 *
 * @uses twentyseventeen_header_style()
 */
function twentyseventeen_custom_header_setup() {

        add_theme_support(
                'custom-header',
                /**
                 * Filters Twenty Seventeen custom-header support arguments.
                 *
                 * @since Twenty Seventeen 1.0
                 *
                 * @param array $args {
                 *     An array of custom-header support arguments.
                 *
                 *     @type string $default-image    Default image of the header.
                 *     @type int    $width            Width in pixels of the custom header image. Default 954.
                 *     @type int    $height           Height in pixels of the custom header image. Default 1300.
                 *     @type string $flex-height      Flex support for height of header.
                 *     @type string $video            Video support for header.
                 *     @type string $wp-head-callback Callback function used to styles the header image and text
                 *                                    displayed on the blog.
                 * }
                 */
                apply_filters(
                        'twentyseventeen_custom_header_args',
                        array(
                                'default-image'    => get_parent_theme_file_uri( '/assets/images/header.jpg' ),
                                'width'            => 2000,
                                'height'           => 1200,
                                'flex-height'      => true,
                                'video'            => true,
                                'wp-head-callback' => 'twentyseventeen_header_style',
                        )
                )
        );

        register_default_headers(
                array(
                        'default-image' => array(
                                'url'           => '%s/assets/images/header.jpg',
                                'thumbnail_url' => '%s/assets/images/header.jpg',
                                'description'   => __( 'Default Header Image', 'twentyseventeen' ),
                        ),
                )
        );
}
add_action( 'after_setup_theme', 'twentyseventeen_custom_header_setup' );

if ( ! function_exists( 'twentyseventeen_header_style' ) ) :
        /**
         * Styles the header image and text displayed on the blog.
         *
         * @see twentyseventeen_custom_header_setup().
         */
        function twentyseventeen_header_style() {
                $header_text_color = get_header_textcolor();

                // If no custom options for text are set, let's bail.
                // get_header_textcolor() options: add_theme_support( 'custom-header' ) is default, hide text (returns 'blank') or any hex value.
                if ( get_theme_support( 'custom-header', 'default-text-color' ) === $header_text_color ) {
                        return;
                }

                // If we get this far, we have custom styles. Let's do this.
                ?>
                <style id="twentyseventeen-custom-header-styles" type="text/css">
                <?php
                // Has the text been hidden?
                if ( 'blank' === $header_text_color ) :
                        ?>
                .site-title,
                .site-description {
                        position: absolute;
                        clip: rect(1px, 1px, 1px, 1px);
                }
                        <?php
                        // If the user has set a custom color for the text use that.
                else :
                        ?>
                .site-title a,
                .colors-dark .site-title a,
                .colors-custom .site-title a,
                body.has-header-image .site-title a,
                body.has-header-video .site-title a,
                body.has-header-image.colors-dark .site-title a,
                body.has-header-video.colors-dark .site-title a,
                body.has-header-image.colors-custom .site-title a,
                body.has-header-video.colors-custom .site-title a,
                .site-description,
                .colors-dark .site-description,
                .colors-custom .site-description,
                body.has-header-image .site-description,
                body.has-header-video .site-description,
                body.has-header-image.colors-dark .site-description,
                body.has-header-video.colors-dark .site-description,
                body.has-header-image.colors-custom .site-description,
                body.has-header-video.colors-custom .site-description {
                        color: #<?php echo esc_attr( $header_text_color ); ?>;
                }
        <?php endif; ?>
        </style>
                <?php
        }
endif; // End of twentyseventeen_header_style().

/**
 * Customize video play/pause button in the custom header.
 *
 * @param array $settings Video settings.
 * @return array The filtered video settings.
 */
function twentyseventeen_video_controls( $settings ) {
        /* translators: Hidden accessibility text. */
        $settings['l10n']['play'] = '<span class="screen-reader-text">' . __( 'Play background video', 'twentyseventeen' ) . '</span>' . twentyseventeen_get_svg( array( 'icon' => 'play' ) );
        /* translators: Hidden accessibility text. */
        $settings['l10n']['pause'] = '<span class="screen-reader-text">' . __( 'Pause background video', 'twentyseventeen' ) . '</span>' . twentyseventeen_get_svg( array( 'icon' => 'pause' ) );
        return $settings;
}
add_filter( 'header_video_settings', 'twentyseventeen_video_controls' );