feat: support css variables for SvgCss (#2459)

# Summary
Feature #2380 

We want to add support for CSS variables when passing them to parse the
SVG XML source function.

## Test Plan
Test app -> src -> Test2380

## Compatibility

| OS      | Implemented |
| ------- | :---------: |
| iOS     |    ✅     		 |
| MacOS   |    ✅        |
| Android |    ✅     	|
| Web     |    ✅		    |

---------

Co-authored-by: Jakub Grzywacz <jakub.grzywacz@swmansion.com>

apps/test-examples/index.tsx

@@ -27,6 +27,7 @@ import Test2327 from './src/Test2327';
 import Test2233 from './src/Test2233';
 import Test2363 from './src/Test2363';
 import Test2366 from './src/Test2366';
+import Test2380 from './src/Test2380';
 import Test2397 from './src/Test2397';
 import Test2403 from './src/Test2403';
 import Test2407 from './src/Test2407';

apps/test-examples/src/Test2380.tsx

@@ -0,0 +1,58 @@
+import React from 'react';
+import {View} from 'react-native';
+import {SvgCss} from 'react-native-svg/css';
+
+const xml = `
+<svg width="200" height="200" id="mySVG" viewBox="0 0 200 200">
+    <style>
+      #mySVG {
+        --my-color: #ff0000;
+        --color: #3e3efe;
+        --green-color: green;
+        --my-gold-stop-color: gold;
+        --my-red-stop-color: red;
+        --my-feFlood-color: green;
+      }
+      #newId {
+        fill: purple;
+        }
+    </style>
+    <defs>
+    <linearGradient id="myGradient">
+      <stop offset="5%" stop-color="var(--my-gold-stop-color)" />
+      <stop offset="95%" stop-color="var(--my-red-stop-color)" />
+    </linearGradient>
+    <filter id="spotlight">
+      <feFlood
+        result="floodFill"
+        x="0"
+        y="0"
+        width="100%"
+        height="100%"
+        flood-color="var(--my-feFlood-color)"
+        flood-opacity="1" />
+      <feBlend in="SourceGraphic" in2="floodFill" mode="multiply" />
+    </filter>
+    </defs>
+    <rect fill="var(--color)" x="40" y="40" width="32" height="32" />
+    <rect id="newId" x="80" y="40" width="32" height="32" />
+    <text fill="var(--color)" x="20" y="100" stroke="var(--green-color)" stroke-width="1" font-size="16">Hello</text>
+    <circle cx="140" cy="55" r="15" fill="url(#myGradient)" />
+
+    <image
+      href="https://static-00.iconduck.com/assets.00/mdn-icon-2048x1806-enhibj42.png"
+      x="0"
+      y="40"
+      width="32"
+      height="32"
+      style="filter:url(#spotlight);" />
+  </svg>
+`;
+
+export default function SvgComponent() {
+  return (
+    <View style={{flex: 1, justifyContent: 'center', alignItems: 'center'}}>
+      <SvgCss xml={xml} height="200" width="200" />
+    </View>
+  );
+}

src/css/css.tsx

@@ -592,6 +592,64 @@ const parseProps = {
  * @author strarsis <strarsis@gmail.com>
  * @author modified by: msand <msand@abo.fi>
  */
+
+function extractVariables(stylesheet: CssNode): Map<string, string> {
+  const variables = new Map<string, string>();
+
+  csstree.walk(stylesheet, {
+    visit: 'Declaration',
+    enter(node) {
+      const { property, value } = node as Declaration;
+      if (property.startsWith('--')) {
+        const variableName = property.trim();
+        const variableValue = csstree.generate(value).trim();
+        variables.set(variableName, variableValue);
+      }
+    },
+  });
+
+  return variables;
+}
+
+function resolveVariables(
+  value: string | CssNode | undefined,
+  variables: Map<string, string>
+): string {
+  if (value === undefined) {
+    return '';
+  }
+  const valueStr = typeof value === 'string' ? value : csstree.generate(value);
+  return valueStr.replace(
+    /var\((--[^,)]+)(?:,\s*([^)]+))?\)/g,
+    (_, variableName, fallback) => {
+      const resolvedValue = variables.get(variableName);
+      if (resolvedValue !== undefined) {
+        return resolveVariables(resolvedValue, variables);
+      }
+      return fallback ? resolveVariables(fallback, variables) : '';
+    }
+  );
+}
+
+const propsToResolve = [
+  'color',
+  'fill',
+  'floodColor',
+  'lightingColor',
+  'stopColor',
+  'stroke',
+];
+const resolveElementVariables = (
+  element: XmlAST,
+  variables: Map<string, string>
+) =>
+  propsToResolve.forEach((prop) => {
+    const value = element.props[prop] as string;
+    if (value && value.startsWith('var(')) {
+      element.props[prop] = resolveVariables(value, variables);
+    }
+  });
+
 export const inlineStyles: Middleware = function inlineStyles(
   document: XmlAST
 ) {
@@ -604,6 +662,7 @@ export const inlineStyles: Middleware = function inlineStyles(
   }
 
   const selectors: FlatSelectorList = [];
+  let variables = new Map<string, string>();
 
   for (const element of styleElements) {
     const { children } = element;
@@ -615,7 +674,10 @@ export const inlineStyles: Middleware = function inlineStyles(
     // collect <style/>s and their css ast
     try {
       const styleString = children.join('');
-      flattenToSelectors(csstree.parse(styleString, parseProps), selectors);
+      const stylesheet = csstree.parse(styleString, parseProps);
+
+      variables = extractVariables(stylesheet);
+      flattenToSelectors(stylesheet, selectors);
     } catch (parseError) {
       console.warn(
         'Warning: Parse error of styles of <style/> element, skipped. Error details: ' +
@@ -636,6 +698,15 @@ export const inlineStyles: Middleware = function inlineStyles(
   // stable sort selectors
   const sortedSelectors = sortSelectors(selectorsPseudo).reverse();
 
+  const elementsWithColor = cssSelect(
+    '*[color], *[fill], *[floodColor], *[lightingColor], *[stopColor], *[stroke]',
+    document,
+    cssSelectOpts
+  );
+  for (const element of elementsWithColor) {
+    resolveElementVariables(element, variables);
+  }
+
   // match selectors
   for (const { rule, item } of sortedSelectors) {
     if (rule === null) {
@@ -667,7 +738,12 @@ export const inlineStyles: Middleware = function inlineStyles(
             const current = priority.get(name);
             if (current === undefined || current < important) {
               priority.set(name, important as boolean);
-              style[camel] = val;
+              // Handle if value is undefined
+              if (val !== undefined) {
+                style[camel] = val;
+              } else {
+                console.warn(`Undefined value for style property: ${camel}`);
+              }
             }
           }
         },