refactor component styles to use type-safe stitches tokens

https://stitches.dev/docs/typescript#stricter-experience

components/HorizontalRule/HorizontalRule.tsx

@@ -1,10 +1,10 @@
-import { styled } from "../../lib/styles/stitches.config";
+import { styled, theme } from "../../lib/styles/stitches.config";
 
 const HorizontalRule = styled("hr", {
   margin: "1.5em auto",
   height: "0.175em",
   border: 0,
-  backgroundColor: "$light",
+  backgroundColor: theme.colors.light,
 });
 
 export default HorizontalRule;