Probably the most common and easily acquired ability in the game is infravision, the ability to see when there’s no light. Aside from being a very common racial ability, rings of infravision can be bought cheaply from Barnum in Arelium, and many guilds grant the infravision spell.
On most minimaps, rooms being seen with infravision only are tinted red. Since some ansi themes color aggressive monsters red by default, this might make it hard to see them. You can fix this problem by changing the coloration with the ansi setting for aggrmons. For example, ‘ansi aggrmons light_red’