I have a Pepperstone account. In the very top right hand corner of the platform there are three horizontal lines. Click on those and you’ll see a slider for light and dark mode. It should update instantly.
PS I know the “Pepper” chatbot can be irritating and you rarely get an answer to your question through it, but you can skip straight to a real person by selecting the menu next to the chat textbox and pressing “transfer to live agent”. Someone usually pops up within a minute or so.