Box util for Stitches CSS in JS

I had a specific border I wanted to use consistently in my web design, so I made a util for Stitches (styling) that took the position the border needed to be in and did the work of styling the border for light and dark mode.

createStitches({
  utils: {
	  box: (sides: Array<"top" | "left" | "bottom" | "right">) =>
	      (sides.length ? sides : sidesOptions).reduce<{
	        [k: string]: string;
	      }>((prevVal, side) => {
	        prevVal[`border-${side}`] = "1px dashed $gray200";
	        if (!prevVal[`.${darkTheme} &`]) {
	          // @ts-ignore
	          prevVal[`.${darkTheme} &`] = {};
	        }
	        // @ts-ignore
	        prevVal[`.${darkTheme} &`][`border-${side}`] = "1px dashed $gray800";
	        return prevVal;
	      }, {}),
	},
});