Please show me how to use an if/else statement in combination with wp_enqueue

I think something like the following should work. Uncomment the var_dump to double check you’re getting the saved theme mod value you’re expecting. If not, check the naming of your customizer setting and that it saved properly.

function mytheme_styles() {

  $theme_dir_uri = get_template_directory_uri();

  // enqueue common styles and scripts
  // wp_enqueue_style( $handle, $src = false, $deps = array(), $ver = false, $media="all" );
  // wp_enqueue_script( $handle, $src = false, $deps = array(), $ver = false, $in_footer = false );

  $header_layout_style = get_theme_mod( 
    'layout_header_desktop', // name of your theme mod
    'regular' // default value, if no saved mod
  );

  // var_dump($header_layout_style);

  if ( 'fullscreen' === $header_layout_style ) {
    // enqueue relevant styles and scripts
    // wp_enqueue_style( 'headerMobileLarge', $theme_dir_uri  . '/assets/css/header-mobile-large.css' );
  } else {
    // enqueue other styles and scripts
    // wp_enqueue_style( 'headerDesktop', $theme_dir_uri  . '/assets/css/header-desktop.css' );
  }

}