What I like about this request, less than the specific ability to disable syncing themes, is the more general ability to set themes per device. As I imagine it, focusing on the setting, rather than the not-syncing might lead to a few different options.
I think this is already possible with Obsidian Sync as well. Although this might override all settings, which is a big downside.
Settings → About → Override config folder
I’ve also seen a few help requests asking for the ability to set styles based on detecting the device. There might be some existing CSS workarounds too.