You already can, hold down on the Like button to add a reaction.

This is planned for v5.2

The dark mode is already supported: Go to settings > Appearance > Dark Mode

That's a great idea! I look into this in a future update.

Is in version 5.1.

You can. Please head to Accounts Manager where you can reorder all posts.

Please give v5.1 beta a go:

https://testflight.apple.com/join/KLqwN1Mb