Opening.bnr is a normal ARC-file with a extra header. The ARC-file contains 3 files, icon.bin, banner.bin, and sound.bin. These .bin-files are compressed using the LZ77 encryption, more information on Wikipedia.
The extra header is very similar (identical?) to the part A of the content.bin file, starting from (and including) the field Zeroes_1, i.e. the second block of part A. See Content.bin_file_structure.