There are currently 4 options shown in the settings menu on the top right of the page.
Dark mode/light mode: changes the colour setting of the website.
Lays as backer's stake/lays as risk: changes the display and input of lay bet liquidity/risk. When set to backer's stake, this is the same as traditional betting exchanges where the input and liquidity in the trading grid is relative to the stake the backer of your order would get. Lays as risk is a new format where the input and liquidity in the trading grid is relative to the total risk of the lay bet.
Football/soccer: shows either "football" or "soccer" in the sport nav menu.
Decimal/American odds: changes the UI odds display between decimal (1.5/3.0) and American odds (-200/+200). This only affects the UI display of odds. Only decimal odds is used in backend calculations. Currently, bet slips must be submitted using decimal odds, regardless of this setting. Clicking a cell with American odds will automatically populate the bet slip with the correct corresponding decimal odds.