More and more applications implement dark mode. It keeps safe your eyes and comforts the working process. Most powerful IDE's have already supported dark mode and this app covers remanning part of the working process - code review and collaboration processes. You can turn on dark mode privately on your profile page.
It's totally open source project. It will be always free. You can improve it by making pr to https://github.com/xprom/bitbucket-dark-mode