Frankly I have no compatibility issues, all the themes I've created work perfectly on my 3DS. I just down themes from the following creators, @FlameSpeedster , @piplupwater, @PedroKeitawa , @miraclaime , and @MartyDreamy. All the themes I've downloaded from these creators never presented any problems.I dumped my DSP firmware when i did the installation here https://3ds.guide/installing-arm9loaderhax
Many ogg files DO work, some do not.