Doesn't game do this already if you hit "H"?

I also think that this one is solved already.

But if there is another solution coming, please also arrange it so that it can be toggled off. I would really really want to keep the classic appearance of Adom.

I really don't like the "H"ighlight myself, but it does serve its purpose appropriately. I agree, this doesn't seem to be a necessary addition.

If it could be done in a prettier way - I'd use it. Currently I'm switch it off because it's kinda ugly.

Yeah highlight mode is ugly but if it was done right, then I would probably like it.

"H" could switch between multiple highlight modes (e.g. the one already implemented, OP suggestion, and none).

a couple options might be:
highlight mode (only outer ring/circumference)
highlight mode (only walls)
darker color out of view
darker colors (only walls)

not really sure how many more colors ascii has available, maybe one or all of those options could have toggles for people to find something comfortable for them. i agree current highlight is a little too washed out for playing but i still use it to check. :>

edit: (only walls) toggle could include trees
another option could be like a crosshair - 4 highlighted tiles, one at end of view range E/S/W/N... or maybe one space before it if the highlight would be distracting.

again not sure if they're available but i guess personal preference for a highlight mode would be making the out-of-view floors/walls(nothing else?) one shade darker.

Then what would be shown in the cavernous level in situation like that:

You see, field isn't always quite round... That's why highlighting only walls or making crosshair won't do the job well... It would be useful only if all visible tiles are highlighted. At least for me.