Our new exhaust fan is smaller than previous leaving us with a gap between the backsplash and the new exhaust fan. Unfortunately more tiles are not an option. They were installed by previous owners, who left us no tiles and probably done around 20 years ago.

Maybe a piece of stainless steel custom from a local fabricator.

Cheaper to buy another box of tiles

What are you going to do to the side of the cabinet, the unexposed part is the original colour

I am also wondering to do the side on the cabinets. I was also considering putting stainless steal on side and back or if the colour on the sides of the cabinets would even out over time. Any suggestions?

Updated post with a picture from farther back.

How old is the kitchen, or was the old range hood as it would take that time for the timber shades to catchup. You could try sanding and resealing both of the inner sides of the cabinets depending on what you use to seal with will change the colour. I would take your photo to a good paint store and ask for their help. A stainless panel could work but would be a different depth to the tiles. could you replace the splash back if it doesn't cover too big an area.

I would like to see more of kitchen including walls to see what matches. Bit from what u have give Take inspiration from your cups and oil bottle or whatever they are. If you can, find tiles of the same size in another shade ideally in colour range of your accessories. The cabinet sides have not had uv exposure so not as dark. Give the parts that have a sand then use a varnish/stain that matches the colour better.