When I try to launch the demo of the game on Linux I get the error: 

Loading Error

Failed to load: img/SumRndmDde/utb/Lightfire.png

There was also this issue with claw.png. Now I fixed the issue, it was because on Linux "c" and "l" are not the same as "C" or "L". So sometimes the game called for lower case and other times it wanted upper case for those two files. I simply have reenamed a copy of the file so there is both in the folder. Not sure if this issue is in the full version or just the demo but for anyone who has this issue here is a workaround until a patch can be made.