There are methods to improve the appearance of wider cracks where you can clean the dirt out and then fill with clear epoxy glue. You would then colour match the tile and paint the crack. But when cracks are only hairlines I'd struggle to think of how you could disguise them.
Painting all the tiles could be a consideration. But in most cases the tiles will need to be replaced but I understand that can be difficult if you can't find the correct tile. - MitchellMc
I had a hairline crack in some tiles at my old house and I rubbed some paint that was the same colour as the tile into the crack and wiped off the excess before it dried and it worked really well at disguising it. Over time (about 6 months or so) it turned dark again but then I just repeated the process. Only takes a couple of minutes. There’s probably a more permanent method but this worked really well for me. - prettyliving