There are a few collectible sets of items in various colors (balloons, candies, feathers, etc.) so it is possible to add another to the set, but there are no immediate plans for one that I'm aware of currently!
The extended feedback stats are a way to view how your feedback evolves overtime (including your feedback activity). You may learn that there are certain days/times when you trade often and see dips or spikes of particular feedback.
For example, on July 1 if you received five positive feedback and then received only one on July 2, you'd see the line dip. If you then received 10 on July 6, you'd see a spike upwards.
We only have one theme at this time so unfortunately this is not possible. We did have the colors change throughout the day before we made the switch to XenForo from vBulletin (e.g.: there would be a nighttime banner and a darker blue background during the night). This is something we're hoping to bring back in some capacity in the future.
If you have Windows 10, I would recommend turning on 'Night Light' or reducing the brightness within your display settings. An application such as flux may also help combat the brightness. Lifesaver for me!