How can I add a background music?
We use a commercial background music in the demo that we are not allowed to redistribute. You can find the purchase link in the SoundCredits.pdf file located inside the Sounds folder. Once you have downloaded the music and imported it into your project (remember to set its Load Type as Streaming, which is the recommended setting for background music), you just need to drag it into the Audio Source component of the BackgroundMusic prefab located in the Prefabs folder.
How can I add a new level to the map?
The level map contains a scrollable list of buttons that are used to enter the levels (in the scene's hierarchy, you can find the list in CanvasLevelMap/ScrollView/Content/Buttons. These buttons are instances of the LevelMapButton prefab, which has a Num Level property that indicates the level number:
You will need to have created the corresponding level with that number via the level editor.
How can I customize the game's popups?
All the popups in the game are prefabs located in the Resources/Popups folder that are dynamically spawned via script as appropriate during the game and have an associated independent script that contains their specific logic. In order to modify them, you can drag them into the Canvas object of a scene (as a child) and apply your custom changes to them as you normally do with prefabs in Unity.
These are all the popups in the game: