YELLOW bits now give off light and some areas will be dark without them. You can not stand on yellow bits. Underground is very dark now, so you need a lantern or a few yellow bits to see. The Hell Arena is lit by the glow of the lava. Rooms start with two lights behind the glass tiles, if you remove them your room will dim (but not be too dark to see).

Don't place more lights than you need.  Too many and the game starts to slow down.