First of all, great work and kudos for getting this far!
It’s not often that people dare to ask for feedback, and even more rarely do you get anything useful. so I’ll try. 
Probably the biggest thing to improve might be the theme. While I’m not a graphic designer myself, something about the colors and design feels a bit off. (I recognize many component use just default styles and that leaves me thing what can we do to improve the out-of-the-box experience. )
Focusing on theming, here are some ideas to explore:
• Try color scheme generators like Coolors or Canva’s palette generator. There are many.
• Use the Lumo Editor to tweak your theme. It’s an old V14 demo but still works quite well. See updated details in the Vaadin docs
• Check out the StarPass demo for some UI component tricks we tried
• Small, useful animations help UX. I had some ideas using Lottie for that: Animation in Vaadin using Lottie
Anyway, I think you’ve started the right way: first make it work, then make it better. Keep up the good work!
Sami