Change how Reader looks
Choose light or dark colours and a serif or sans face for what you read.
Change colours and face
Open the setting
Open Settings → How your pages look.
Choose colours
Under Colours, pick Follow my system, Light or Dark.
Choose a reading face
Under Reading face, pick Sans serif or Serif, then press Save.
Where it applies
Both take effect on the next page and follow you to any browser you sign in on. The face is used for posts and articles; menus and controls stay as they are.