At the back of the Play Screen during a game is one of several various backgrounds, which typically depends on the current song's pack. Additionally, there are certain songs with unique backgrounds.
This page lists all backgrounds currently in the game. Internal names are in sky blue for Light-side backgrounds, and violet for Conflict-side backgrounds.Whether a background is Light-side or Conflict-side is shown next to the names.
These are backgrounds intended for individual song packs.
lethaeus was initially intended solely for Lethaeus (according to its internal name), but was later also applied to Nhelv. The reason for this is unknown.
Certain generic backgrounds are unique to only one song (some being deprecated in a later update). These backgrounds are listed in Misc Trivia.
Byd_light and Byd_conflict don't have specific tracks. Instead, they use whatever track the song in question would normally use. (eg. Playing a song that uses the mirai_dark background will use the black track and a song that uses the nijuusei_conflict_b background will use the dark_nijuusei track)
Likewise, Kanae's skill will also affect the track colour (playing a light side Sunset Radiance song at night will result in the dark track being used).