This setting is only available in Timesheet and Planner for Jira issue view.
Use this setting in Jira issue view to switch between the different modes available, like Light, Dark, Original, Match System themes.
Dark mode is only available inside the Jira issue view and not in any Tempo apps yet.
For example Tempo Gadgets, My work, etc.
To set the theme:
Click on your profile picture in the top left corner of your screen.
Click Theme
Select Dark or Light mode.