Quote display different then in Block Editor

Hello,

when inserting a Quote in Block Editor, it shows a thin grey line at the left. But on Frontpage, the line is black.

Why is that and how can i set the color for this line?

PS: I cannot link because the post is still a draft
PPS: I have activate the Gutenberg Styles with this code:

add_action( 'after_setup_theme', function() {
    add_theme_support( 'wp-block-styles' );
} );