Hi @BruceS,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
It's a little bit concerning if the cracking has begun to spread from one tile to another. This would indicate that there is an issue along these cracks, likely from a void beneath the tiles. The only long-term solution would be to remove the tiles and replace them, rectifying whatever issue is beneath them that has caused the cracking.
Unfortunately, with hairline cracks like this, it is a bit difficult to actually make any significant repair to them. If you are simply looking for a short-term aesthetic fix, you could head to your local store and grab some paint colour cards, then take them home and select the colour that is closest to your tile. You could then get a sample pot of that colour mixed up and use a rag to rub a small amount into the crack, then, while it is still wet, clean it up with a damp rag to remove any paint from the surface of the tiles.
Unfortunately, this is about the best you can do with hairline cracks. It should lessen the visibility of the crack, but it will not fix it.
Let me know what you think and if you have any further questions, please don't hesitate to ask.
Jacob
