The consistency principles has been used well on the main menu page.
Each button element is sized equally as the others and the different distance between the upper buttons and the lower "quit" button fullfills the UI principles in terms of hierarchy.
I especially appreciate the color palette for the background as it fits the theme really nicely.
The happy atmosphere is well conveyed through the color choice. The black font on the pink buttons makes it more accessible too.
The back panel enhances the priority of the UI elements as it makes the background more transparent. It helps with user control as it reduces the misclicking on accident on other menu objects.
One thing I would improve is the position of the main menu button on the second scene. It is not really consistent with the overall theme.