Navigate to Settings by clicking the hamburger menu in the top left and selecting "Settings".
Under "Assistant", select "Appearance".
This will take you to the assistant appearance page. The settings are on the left side and a preview of the assistant is on the right.
Under "Preview", you can click between "Question card" and "Solution card" to change which preview is displayed.
To use a predefined color theme, under "Assistant appearance" click "Select a theme" and then click the desired theme.
To instead specify individual colors, under "Assistant appearance" click "Select colors". This will allow you to specify the colors used by the assistant.
For each color, specify the desired hex code. The preview will update with any change you make.
When you are satisfied with your assistant's appearance, click "SAVE".