In the Airtech theme, there is a blue line between menu items.
It’s actually an image, if you want to remove it, Please enter below CSS in wp-admin/appearance/customise/additionalCSS box.
.main_menu .navbar .nav.navbar-nav li a:before {
display: none;
}
If you want to change it, you will need to get the desired line as the image created. You can use Photoshop or Online tools like https://photoshoponline.pro/ to create one easily, after the image is ready, upload it to wp-admin/media/add new. Keep its Absolute URL handy and place it in the below code and, enter below CSS in wp-admin/appearance/customise/additionalCSS box.
.main_menu .navbar .nav.navbar-nav li a:before {
background: url("https://path_to_the_image.jpg") no-repeat scroll center center;
}