Click on the user icon at the top right hand corner of the page. This may be a letter or a photo.
A menu is displayed with a User icon on the right of the header. Click this.
Another menu is displayed. Select Preferences.
From the Preferences menu select Interface.
Options to change the theme and colour scheme are displayed. The changes will be applied when Save is clicked. Note that Save is a long way down the page, with lots of blank space.