Hi,
First of all, make sure this file has write acces:
/templates/flexible_darkmart/config
(This is a file without file extension, and it stores all configuration settings. Fail to "re-write" this file depends on new settings will give you the "save failure" message as you have experienced.)
If this file doesn't exist at all, then rename this file (manually):
/templates/flexible_darkmart/config.default
to
/templates/flexible_darkmart/config
CHMOD 644 is enough but also try CHMOD 755
Secondly try a different browser. Safari or IE. This might be also a browser issue.
If still no luck, let me know.
Regards.