Overview
I use Atlassian services every day as part of my work. In particular, I make extensive use of Bitbucket, Confluence, and Jira. At the time of this writing, Atlassian does not support dark mode, at least not at the level of an individual user, which is a bummer.
To overcome this I found a workaround that works for me. I am mainly using the Firefox browser on Ubuntu. There is a Firefox plugin (aka, add-on) called Stylus which provides a set of themes specific to a domain. Stylus has a number of themes for atlassian services. Specifically, I use the "Bitbucket Dark Mode" theme for Bitbucket, and I use the "Conway - Atlassian" theme for Confluence and Jira. They are not perfect but they mostly work, plus you can turn these themes on and off seamlessly.