`:root { font-size: var(--size) }` sets nothing a `rem` reads: `1rem` stays 14

Start with the reported reproducer, src/__tests__/native/root-font-size-through-variable.test.tsx, and trace how root font-size values and custom properties are handled when inlineVariables: false. Compare the passing literal 16px case with the failing var(--size) case, then run the reported Jest test; done means both cases produce fontSize: 16 for 1rem.

Before submitting a new issue

• I tested using the latest version of the library, as the bug might be already fixed.
• I tested using a supported version of react native.
• I checked for possible duplicate issues, with possible answers.

Bug summary

rem is the root element's computed font size (css-values-4), whatever spells it. :root { font-size: 16px } sets the size a 1rem reads; :root { font-size: var(--size) } with --size: 16px, read through a variable the runtime keeps, sets nothing, so 1rem renders the 14 the library assumes without a root size. Tailwind's theme declares its sizes as custom properties, so a root size held by one is the ordinary case.

The written root passes; through a variable the element receives { fontSize: 14 }.

Library version

main at 2421ea7 (0.0.0-nightly.2421ea7)

Environment info

System:
  OS: Windows 11 10.0.26300
  CPU: "(32) x64 AMD Ryzen 9 9950X3D 16-Core Processor          "
  Memory: 155.26 GB / 255.67 GB
Binaries:
  Node:
    version: 26.12.1
    path: C:\Program Files\nodejs\node.EXE
  Yarn:
    version: 4.9.2
    path: C:\Users\<user>\AppData\Roaming\npm\yarn.CMD
  npm:
    version: 12.2.1
    path: C:\Program Files\nodejs\npm.CMD
  Watchman: Not Found
SDKs:
  Android SDK:
    API Levels:
      - "33"
      - "35"
      - "35"
      - "35"
      - "36"
      - "37"
    Build Tools:
      - 35.0.0
      - 36.0.0
      - 36.1.0
      - 37.0.0
    System Images:
      - android-26 | Google APIs Intel x86_64 Atom
      - android-28 | Google APIs Intel x86_64 Atom
      - android-29 | Google APIs Intel x86_64 Atom
      - android-30 | Google APIs Intel x86_64 Atom
      - android-31 | Google APIs Intel x86_64 Atom
      - android-32 | Google APIs Intel x86_64 Atom
      - android-33 | Google APIs Intel x86_64 Atom
      - android-34 | Google APIs Intel x86_64 Atom
      - android-35 | Google APIs Intel x86_64 Atom
      - android-36.1 | Google Play Intel x86_64 Atom
      - android-36.1 | Pre-Release 16 KB Page Size Google Play Intel x86_64 Atom
      - android-37.0 | Google APIs Intel x86_64 Atom
    Android NDK: Not Found
  Windows SDK: Not Found
IDEs:
  Android Studio: AI-262.9437.185.2621.16467767
  Visual Studio: Not Found
Languages:
  Java:
    version: 17.0.20.1
    path: C:\Program Files\Microsoft\jdk-17.0.20.101-hotspot\bin\javac.EXE
  Ruby: Not Found
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.86.3
    wanted: 0.86.3
  react-native-windows: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: Not found
  newArchEnabled: Not found
iOS:
  hermesEnabled: Not found
  newArchEnabled: Not found

Steps to reproduce

1. Clone the reproducer branch, which is main at 2421ea7 with one file added, src/__tests__/native/root-font-size-through-variable.test.tsx, and install:

   git clone --branch repro/524-root-font-size-through-a-variable-sets-no-rem https://github.com/YevheniiKotyrlo/react-native-css
   cd react-native-css
   yarn install
   

2. Run yarn jest src/__tests__/native/root-font-size-through-variable.test.tsx.

src/__tests__/native/root-font-size-through-variable.test.tsx:

import { render, screen } from "@testing-library/react-native";
import { Text } from "react-native-css/components/Text";
import { registerCSS, testID } from "react-native-css/jest";

test.each([
  ["written", ":root { font-size: 16px; } .a { font-size: 1rem; }"],
  [
    "through a variable",
    ":root { --size: 16px; font-size: var(--size); } .a { font-size: 1rem; }",
  ],
])("with the root's font size %s, 1rem is 16", (_, css) => {
  registerCSS(css, { inlineVariables: false });
  render(<Text testID={testID} className="a" />);
  expect(screen.getByTestId(testID).props.style).toStrictEqual({
    fontSize: 16,
  });
});

Expected: :root { font-size: var(--size) } over --size: 16px makes 1rem 16, as :root { font-size: 16px } does.

Actual: 1rem stays 14.

FAIL src/__tests__/native/root-font-size-through-variable.test.tsx
  √ with the root's font size written, 1rem is 16 (17 ms)
  × with the root's font size through a variable, 1rem is 16 (6 ms)
  ● with the root's font size through a variable, 1rem is 16
    expect(received).toStrictEqual(expected) // deep equality
    - Expected  - 1
    + Received  + 1
      Object {
    -   "fontSize": 16,
    +   "fontSize": 14,
      }
      12 |   registerCSS(css, { inlineVariables: false });
      13 |   render(<Text testID={testID} className="a" />);
    > 14 |   expect(screen.getByTestId(testID).props.style).toStrictEqual({
         |                                                  ^
      15 |     fontSize: 16,
      16 |   });
      17 | });
      at toStrictEqual (src/__tests__/native/root-font-size-through-variable.test.tsx:14:50)
Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 passed, 2 total
Snapshots:   0 total
Time:        1.011 s, estimated 2 s
Ran all test suites matching /src\\__tests__\\native\\root-font-size-through-variable.test.tsx/i.

Reproducible example repository

https://github.com/YevheniiKotyrlo/react-native-css/tree/repro/524-root-font-size-through-a-variable-sets-no-rem