Previously, assembly of the theme paths for custom paths (based on
`XDG_DATA_DIRS`) was performed using `g_strconcat`, which produced erroneous
file paths with missing path separators due to a typo in the code.
Instead, make use of `g_build_filename` to assemble the theme paths from
`XDG_DATA_DIRS` to look for custom themes, which automatically introduces
host-specific path separators between individual path components. This
introduces another temporary variable to ensure that the filename (which is
itself assembled from a name and the file ending) isn't mangled by introduction
of a path separator between these two "parts".
Andreas Hartmann
1c53464090