always use forward slashes in relative theme paths

package.json

@@ -216,6 +216,7 @@
     "rollup-plugin-terser": "^2.0.2",
     "sass-loader": "7.1.0",
     "script-ext-html-webpack-plugin": "2.0.1",
+    "slash": "^3.0.0",
     "source-map": "0.7.3",
     "source-map-loader": "0.2.4",
     "string-replace-loader": "^2.1.1",

webpack/helpers.js

@@ -1,6 +1,6 @@
 const path = require('path');
 const fs = require('fs');
-
+const slash = require('slash');
 
 const projectRoot = (relativePath) => {
   return path.resolve(__dirname, '..', relativePath);
@@ -68,7 +68,7 @@ const globalCSSImports = (env) => { return [
 const getThemedPath = (componentPath, ext) => {
   const parsedPath = path.parse(componentPath);
   const relativePath = path.relative(srcPath, parsedPath.dir);
-  return path.join(relativeThemePath, relativePath, `${parsedPath.name}.${ext}`);
+  return slash(path.join(relativeThemePath, relativePath, `${parsedPath.name}.${ext}`));
 };
 
 const themedTest = (origPath, extension) => {

yarn.lock

@@ -9514,6 +9514,11 @@ slash@^1.0.0:
   resolved "https://registry.yarnpkg.com/slash/-/slash-1.0.0.tgz#c41f2f6c39fc16d1cd17ad4b5d896114ae470d55"
   integrity sha1-xB8vbDn8FtHNF61LXYlhFK5HDVU=
 
+slash@^3.0.0:
+  version "3.0.0"
+  resolved "https://registry.yarnpkg.com/slash/-/slash-3.0.0.tgz#6539be870c165adbd5240220dbe361f1bc4d4634"
+  integrity sha512-g9Q1haeby36OSStwb4ntCGGGaKsaVSjQ68fBxoQcutl5fS1vuY18H3wSt3jFyFtrkx+Kz0V1G85A4MyAdDMi2Q==
+
 slide@^1.1.3:
   version "1.1.6"
   resolved "https://registry.yarnpkg.com/slide/-/slide-1.1.6.tgz#56eb027d65b4d2dce6cb2e2d32c4d4afc9e1d707"