Themes are characteristics of levels which change whether Group Objects are created when loading a layout. For example, by placing themed Groups in the environment layout you can create weather which has a random chance of appearing when the level is created. Level Themes can be set on Dungeons or Level Rule Sets. The theme definitions are in /media/levelthemes/ . They're stored in separate files so that people can more easily mod in level themes.

Themes can be created using the Themes Editor.

A data group in a dungeon or rule set will look like this:

[THEMES]
	[GROUP]
		[THEME]
			<INTEGER64>ID:-5068153475201429025
			<INTEGER>WEIGHT:1
		[/THEME]
		[THEME]
			<INTEGER64>ID:-6023112609921429025
			<INTEGER>WEIGHT:1
		[/THEME]
	[/GROUP]
	[GROUP]
		<INTEGER64>REQUIRED_THEME:-6023112609921429025
		[THEME]
			<INTEGER64>ID:5955778803944067551
			<INTEGER>CHANCE:50
		[/THEME]
	[/GROUP]
	[GROUP]
		<INTEGER64>REQUIRED_THEME:-6023112609921429025
		[THEME]
			<INTEGER64>ID:8963748009504018911
			<INTEGER>WEIGHT:1
		[/THEME]
	[/GROUP]
	[GROUP]
		[THEME]
			<INTEGER64>ID:2531681805941215711
			<INTEGER>WEIGHT:1
		[/THEME]
	[/GROUP]

[/THEMES]

They're not much fun to look at in data since all the themes are referenced by ID, but in the Dungeon Editor and Rule Set Editor this same data will look like this:

• NOT_THEME
  • (INTEGER64) - If this theme is already defined, this group will not roll
• REQUIRED_THEME
  • (INTEGER64) - If group will only roll if this theme is already defined
• PICKS
  • (INTEGER) - Number of themes to pick from this group

• ID
  • (INTEGER64) - The potential theme's ID
• WEIGHT
  • (INTEGER64) - The weight for this chance when rolling against other themes. If the higher the weight, the more likely this theme will be selected.
• CHANCE
  • (INTEGER) - Percent chance to use this theme.

NOTE: Groups are rolled in the order they are listed in data

NOTE: If WEIGHT and CHANCE themes are put in the same group, CHANCE themes will be rolled first, then any WEIGHT themes will be rolled to fill in any remaining picks.

Themes - Runic Wiki