This is not meant to be an apokalyptic volcanic tileset with lots of destruction. Its more a peaceful island with volcanic activty.
If the ground is too dark for you, you should raise your gamma settings. For this MG has a commandline switch called "--set-gamma" . Its only documented in svn, but I think already available in 3.6.0.3.
Next version will have this in the options menu. (Its already implemented I just need to check in) .
Set this to numbers between 0.0 up to 3.0 and the night/day should not be so dark anymore. If you know a good value you can put this into your glestuser.ini too (GammaValue=1.1).
( About the coffins: I was running out of ideas/motivation and simply used them from the Hell tileset
)