Your appearance settings are in the settings menu. Click the gear icon at the top of the left sidebar, to the right of the search and bell icons, to open it. Both settings below are saved to your account, so they follow you to any browser you sign in from.
Dark mode
Flip the Dark mode switch to change between the dark and light theme. The change applies straight away.
Expanded text
By default the trees in the left sidebar show each item title on a single line and cut off long titles. Flip the Expanded text switch to wrap titles so you can read them in full. The setting also applies to the trace titles and test run names listed in the item details sidebar, and to attribute values there, which are shown in full rather than cut off.
Other entries in the menu
The same menu holds the organization switcher (see Switching Organizations), Organization settings, My account, Help & feedback (see Getting help) and Sign out. System admins also see System administration here.