Add note about units in theming

Author: jxnblk

packages/docs/src/pages/theming.mdx

@@ -58,6 +58,13 @@ Core typographic values can be stored in the following theme keys:
 - `lineHeights`
 - `letterSpacings`
 
+<Note>
+
+Values defined as numbers are converted to pixel units where appropriate.
+For other units, use string values, e.g. `'1em'`.
+
+</Note>
+
 ```js
 // example theme object
 {