First of all, thanks for your comments and ideas. I would like to say that we will take all comments into consideration during the ongoing development of the game.
1. Initially, we wanted to design a tutorial section, but due to time constraints, we wanted to publish our game, albeit with its current deficiencies. But we will definitely design a tutorial section and we will try to make the first levels a little easier.
2. For the art design part, We mostly used free sprite assets (for the character, tile set etc.). In later we are planning to switch all the sprites with new ones designed by our teammates. So after that I believe the background color complexity problem is going to be fixed.
3. For the portal placement part, I tried to code the platform placement mechanic in such a way that when you press the area a certain distance above the platforms, a portal will be placed on the platform closest to the point you clicked on. As the picture below, when you click the red areas the portal will be placed the closest platform. If you think the portal placement mechanics is not good in terms of gameplay, I can try to find a better way to handle this placement mechanics.