refact: extract typography constants

src/globalStyles.tsx

@@ -1,12 +1,17 @@
 import { createGlobalStyle } from "styled-components"
+import {
+  BASE_FONT_SIZE,
+  BASE_LINE_HEIGHT,
+  fontFamily,
+} from "./design/typography"
 
 export const GlobalStyles = createGlobalStyle`
   html {
-    font-family: Oxygen;
+    font-family: ${fontFamily.body};
   }
 
   body {
-    font-size: 18px;
-    line-height: 25px;
+    font-size: ${BASE_FONT_SIZE}px;
+    line-height: ${BASE_LINE_HEIGHT}px;
   }
 `