Hello Simon234,
Thanks for reaching us. As guys here correctly stated we focus mainly on security and privacy features, but we also take in mind comfort of our users.
I am not sure about which theme you are talking. I think you are talking about windows dark mode. I will rather answer you for both options that i know about.
1) Windows 10 dark mode for applications. This functionality will be included in Avast Secure Browser soon. We had a bug that caused this feature to stop working, but we already fixed it and it will be part of the next v 77.0 release.
2) Themes from Chrome store. We plan to support themes but it’s not an immediate priority.