Add music theming.

README.md

@@ -88,6 +88,8 @@ Enum bitmask of the types of growables that will spawn.
 Customize the background to look like the background of `background_theme`.
 * `floor_theme` THEME \
 Customize the floor textures to look like the floors of `floor_theme`.
+* `music_theme` THEME \
+Customize the music to play the music for `music_theme`.
 * `post_configure` function(CustomTheme, Subtheme) \
 Function that allows additional configuration of the CustomTheme that was created from the properties.
 

level_sequence.lua

@@ -555,6 +555,12 @@ local function update_state_and_doors()
 	state.world = current_level.world or index_of_level(current_level)
     state.level = current_level.level or 1
 
+    if current_level.music then
+        state.theme = current_level.music
+    elseif current_level.music_theme then
+        state.theme = current_level.music_theme
+    end
+
     if sequence_state.keep_progress then
 		-- Setting the _start properties of the state will ensure that Instant Restarts will take
         -- the player back to the current level, instead of going to the starting level.