Thank you for the detailed feedback. I am glad you like my game!
My goal was to create a full playable game with a (realy short) story so that the player now what's going on and what he/she is supposed to do. Of course I had tons of ideas I wanted to implement but 10 days is a rater short time frame. So I had to focus on one thing and make that one thing as good as possible with the risk that it gets repetitive and quickly boring. I see this game as a prototype to test the gem mechanics to get some feedback to see how players like it.
I wrote a short concept of how this prototype could turn into a full game - based on this core mechanic. I already started to work on the new version.
I noticed the lines between background tiles as well. I am actually using the Pixel Perfect Camera but I am also using the Cinemachine package from Unity for camera moving and camera bounding box. And instead of the default renderer I am using the Universal Render Pipeline with the new 2D renderer. I guess the issue came in the combination of those things. I will fix it eventually. :-)