Apply trim, painted to match railing color, over tile edges.

Not much, if anything, that you can do.

A trim board would have to be very wide (and out of scale, IMO). Also, the angle would mean that a trim board would slice over a good portion of the switch box and have to have an ugly cut-out. And there is the added problem of how to fasten the board over tile and fill the gap at the top of the board that will be created because, I'm assuming, the tile & wall are in two different planes.

Since you say that you like it (but not the jagged edge), I would just live with it as is until I tired of it and then remove it.

Darker tile cut at an angle to fill in the step part ?

The trim board is probably the only workable solution aside from tearing out the tile. Hard to believe someone thought this was a good idea for tile application.

Remove those tiles for sure how do they connect to the rest of the house there is no way those work on the wall .

OP states they like it. What about hanging a series of matching framed artwork above the stepped tiles?

And/or painting the wall above in a grey that would make the tile contrast less with the wall?