refactored text styles, not enough shared

app/styles/dimensions.ts

@@ -0,0 +1,14 @@
+import { StatusBar } from 'react-native'
+
+const header = 56
+const tabBar = 54
+
+const top = () => header + (StatusBar.currentHeight || 0)
+const bottom = () => tabBar
+
+export default {
+  header,
+  tabBar,
+  top,
+  bottom,
+}