Theme styling for plugins

Your two options are to add your own stylesheet to redefine the plugin’s styles, or remove the plugin’s css file entirely by dequeueing the stylesheet (assuming the plugin has correctly added the stylesheet via the API)

function wpd_my_theme_css() {
    // dequeue plugin css with same handle it is enqueued with
    wp_dequeue_style( 'plugin_css_handle' );
}
// priority must by a higher number than the css is added on
add_action( 'wp_enqueue_scripts', 'wpd_my_theme_css', 9999 );

You’ll have to look at the specific plugin’s code for a definitive answer on what’s possible here.