fix: use :root font-size as compile-time rem base (#315)

When no explicit inlineRem option is passed, the compiler now scans CSS
for :root { font-size: Npx } and uses that value as the rem multiplier
instead of hardcoding 14. This makes the documented v5 approach for
overriding rem actually work.

Closes nativewind/nativewind#1650

Author: danstepanov

src/__tests__/native/units.test.tsx

@@ -126,6 +126,28 @@ test("rem - inline override", () => {
   });
 });
 
+test("rem - css root font-size override", () => {
+  registerCSS(`
+    :root { font-size: 16px; }
+    .my-class { font-size: 10rem; }
+  `);
+
+  const { result } = renderHook(() => {
+    return useNativeCss(View, { className: "my-class" });
+  });
+
+  expect(result.current.type).toBe(VariableContext.Provider);
+  expect(result.current.props.value).toStrictEqual({
+    [VAR_SYMBOL]: true,
+    [emVariableName]: 160,
+  });
+
+  expect(result.current.props.children.type).toBe(View);
+  expect(result.current.props.children.props).toStrictEqual({
+    style: { fontSize: 160 },
+  });
+});
+
 test("rem - css override", () => {
   registerCSS(
     `

src/compiler/compiler.ts

@@ -84,17 +84,28 @@ export function compile(code: Buffer | string, options: CompilerOptions = {}) {
 
   const vars = new Map<string, UniqueVarInfo>();
 
+  // Determine the effective rem multiplier for compile-time inlining.
+  // If inlineRem is explicitly set, use that. Otherwise, scan for
+  // :root { font-size: Npx } in the CSS to allow CSS-based configuration.
+  let effectiveRem: number | false = options.inlineRem ?? undefined!;
+  if (effectiveRem === undefined) {
+    const css = typeof code === "string" ? code : code.toString();
+    const match = css.match(/:root\s*\{[^}]*font-size:\s*([\d.]+)px/);
+    effectiveRem = match?.[1] ? parseFloat(match[1]) : 14;
+  }
+
   const firstPassVisitor: Visitor<CustomAtRules> = {};
 
-  if (options.inlineRem !== false) {
+  if (effectiveRem !== false) {
+    const remMultiplier = effectiveRem;
     firstPassVisitor.Length = (length) => {
-      if (length.unit !== "rem" || options.inlineRem === false) {
+      if (length.unit !== "rem") {
         return length;
       }
 
       return {
         unit: "px",
-        value: round(length.value * (options.inlineRem ?? 14)),
+        value: round(length.value * remMultiplier),
       };
     };
   }