How could I change the space between the thumbnail?
I added a padding-right: 10px CSS to the thumbnails, but In this case there is a 10px space next to the thumbnail on the right (I attach an iamge to better understanding).
The padding is added to the right side of each thumbnail. So when you increase its value it is displayed for the last one. This is how the padding works.
You can try adding padding to the left and make them equal. Otherwise, it requires customizations.