Favicon and Custom CSS

This small image shows up in browser tabs, bookmarks, and some mobile shortcuts. A square PNG around 512 × 512 pixels is the recommended size -- it gets scaled down automatically for the many smaller sizes different browsers and devices actually use, so starting large keeps it looking sharp everywhere.

Custom CSS lets you write raw styling code that applies across the whole site. It is a genuine escape hatch for something the Colors, Fonts, and section options above truly cannot do -- but reach for those first every time, since they are built to work together and stay compatible as the platform updates. Custom CSS you write yourself is not automatically kept in sync with anything, and a future design change elsewhere could interact with it in ways that are hard to predict. If you are not sure whether you need it, you probably do not yet.