gui: Always initialize theme.

gui: Always initialize theme.

parse_config_file_or_die() contains the following bug: If the
configuration file ~/.paraslash/gui.conf does not exist, the current
code not only skips the gengetopt config file parser (which is correct)
but also omits to call theme_init(), which is incorrect.

The bug was introduced recently in commit 18d37e (Unify config
file parsing). Fix it by moving the call to theme_init() to
the bottom of the function so that it is called if and only if
parse_config_file_or_die() succeeds.