I see some type errors in declarations.d.ts file after extending HTMLAttributes with css prop. Could you look at this?

Type cast is not a solution here, because we are losing type information. props are not only SpinnerProps — they are SpinnerProps, StyledSpinnerProps and HTML div element props.

But, it's a fact that styled-components and TypeScript doesn't work well together, unfortunately.
There are types mismatches, for example, type of prop color from styled-system conflicts with div element's color.

Here is how to deal with it (in order to preserve type information):

1. Crate component Box and interface BoxProps. You can move it to its own folder.

interface BoxProps extends Omit<React.ComponentPropsWithRef<'div'>, 'color'> {}
const Box = ('div' as any) as React.FC<BoxProps>;

2. Create an interface StyledSpinnerOwnProps. It should be an interface, not a type, because again, using type and & results in types conflicts.

interface StyledSpinnerOwnProps
  extends ColorProps,
    BorderProps,
    TypographyProps,
    LayoutProps,
    SpaceProps,
    ShadowProps {}

3. Declare StyledSpinner:

export const StyledSpinner = styled(Box)<StyledSpinnerOwnProps>`
  position: relative;

  ${color}
  ${border}
  ${typography}
  ${layout}
  ${space}
  ${shadow}
`;

4. Export StyledSpinnerProps as:

export interface StyledSpinnerProps extends BoxProps, StyledSpinnerOwnProps {}

Check the whole code here.

5. Then in index.js use this type:

export interface SpinnerProps extends StyledSpinnerProps {
  size: string;
}

Great! Approving.