The old dark theme is available here: Official classic & dark theme of pre-3.6 Audacity as custom theme
As for the scrollbars and menus, those are older native Windows components which we cannot control, and which just always are light. This also isn’t restricted to Audacity, it even happens to the Windows components Microsoft doesn’t care about. For example, file properties are using the same system:
We will fully move away from using native Windows elements for Audacity 4.