Twenty Seventeen: Allow child themes to easily extend custom color patterns

By adding a filter, child themes can add additional selectors onto the custom color scheme CSS. Like so:

{{{
// Add child theme selectors for color schemes.
function dynamic_seventeen_custom_colors_css( $css, $hue, $saturation ) {
	$css .= '
	.colors-custom .content-menu > article:not(.has-post-thumbnail),
	.colors-custom .content-menu > section:not(.has-post-thumbnail) {
		border-top-color: hsl( ' . $hue . ', ' . $saturation . ', 87% ); /* base: #ddd; */
	}';
	return $css;
}
add_filter( 'twentyseventeen_custom_colors_css', 'dynamic_seventeen_custom_colors_css', 10, 3 );
}}}

Props celloexpressions.

Fixes #38949.

Built from https://develop.svn.wordpress.org/trunk@39386


git-svn-id: https://core.svn.wordpress.org/trunk@39326 1a063a9b-81f0-0310-95a4-ce76da25c4cd