There are plenty of dark themes available (settings > style configurator > select theme…), but, as far as I know, all the dark themes are only affecting the editor panes while the UI maintains standard windows coloring.
If you would like the ability for the UI to be dark as well, that will have to be a request that goes to the developer. While the developer posts here occasionally, this is not where bug / feature request tracking occurs. Where to go for feature requests is described in this FAQ. Please remember to search the existing issues, because the chances are, someone else has already requested a dark-UI option – if so, you will want to lend your support and/or reasoning to that existing ticket, rather than creating a new one.