You can use the :not
selector like this:
body:not(.page-id-123):not(.page-id-456) {
background-image: linear-gradient(0deg,rgba(36,55,67,.76),rgba(36,55,67,.76)),url(https://cdn.shortpixel.ai/client/q_glossy,ret_img/http://www.historiadeloscomics.com/wp-content/uploads/2019/01/marvel-1641554_1920.jpg);
}
Just replace the 123
and 456
with the actual page ID that you would like to replace.
The other method would be to target specific page that you do want the background image/overlay:
body.page-id-123 ,body.page-id-456 {
background-image: linear-gradient(0deg,rgba(36,55,67,.76),rgba(36,55,67,.76)),url(https://cdn.shortpixel.ai/client/q_glossy,ret_img/http://www.historiadeloscomics.com/wp-content/uploads/2019/01/marvel-1641554_1920.jpg);
}