Typical browser caches all CSS and JS files to avoid unneeded download. In WordPress, this has a big effect on your changes to  style.css file. These changes will not be immediately visible to old visitors.

Add the following  code to your functions.php. It appends version number to the URL (in query string). Changing the version number forces a browser to re-download style.css.

function my_wp_default_styles_scripts($styles) {
    $my_theme = wp_get_theme();
    $styles->default_version = $my_theme->get( 'Version' );
add_action("wp_default_styles", "my_wp_default_styles_scripts");
add_action("wp_default_scripts", "my_wp_default_styles_scripts");

It’s a simple fix, but it’s not well documented in WordPress.