The code really isn't set up to easily make changes like that,  but it's possible.

Hazards are stored at line 590, "if tils(px,py,px+7,py+7, [tile index] ) then die() end" will turn [tile index] into a hazard.

The heart is not actually an animated tile but a sprite drawn at the level's end. TIC-80 does have support for animated tiles through writing special "remap" functions, but this game does not use one.

Making a specific colour deadly is possible but much more difficult than making a tile deadly, I'd stick with the method I described above.

If you're this interested in modding Celesteditor have you considered trying to make your own game? Making something like this isn't as complicated as it might seem using a tutorial like this one, and it would mean that you aren't restricted to building off of someone else's game.