Hi @Dunky,
This is likely due to the gradual rollout of the new Magnetic design preventing a return to the older theme. There was previously the option to opt-in to it as an early access feature, but this looks to have been removed as we transition to Magnetic design as the default theme.
Please let me know if Support has provided any further context on this too!
If you found this post helpful, please give it kudos. If my answer solved your problem, click "accept as solution" so that others can benefit from it.