Can you stain ceramic tile

Not all finishes on ceramic tiles are glossy; some are flat.

Was the tile originally glossy?

I would suggest that you first give it a very thorough cleaning and polishing to see if this restores its shine.

Stain will not hold to tile.

Agree with William. the answer is No.

How about alcohol inks, then varnish. I haven't tried it on the floor but l have on some tiles take the ink and after varnish have used as coasters. Not sure how well they will hold upto all the cleaning and water though.

Do you have spare tiles to test first.

I wouldn't trust stain to hold on tile. But you could paint it- https://www.hometalk.com/diy/paint/how-to-paint-a-floor-with-a-tile-stencil-31754162

Might be better to use self adhesive Tiles over. or Transfers??