you aren't a true computing major until you use dark themes. be the top dog in the programming lab.
eyebleed........................... try some base16. a different font too pls, serif sucks
(base16-default-dark and consolas in sublime)
also, fyi
first defining a variable in a block and then accessing it outside of that block is a strange, often error-prone move. wouldn't suggest it
and you don't really need to comment explain things that are obvious, like that first comment. the variable name and prompt string are more than enough to understand its purpose
also python 3 string formatting is good
you could actually make it even nicer as of 3.6......
print(f"ERROR: {file_name} not found.")