I really liked the overall theme of the game, gives a cute with zombie theme. I also like the theming that you have done with the colours, which really complement each other and the font as well. I have some feedback on user experience as a general user.
It is evident that you have well-implemented UX principles. I can clearly see that you have tried to create a hierarchy in the menu scene by creating a panel and making the font and button larger, but one thing that I can see is that there might not be enough contrast between the menu button text and button colour which might cause usability issue in some use cases. Besides that the menu screen is perfect.
The font type that you have chosen is perfect for the theme of the game, and the backdrop really creates a good user experience for the game theme that you are aiming for. Overall, I had a very good user experience using your grey box project.