diff --git a/services/src/main/java/org/keycloak/theme/ResourceLoader.java b/services/src/main/java/org/keycloak/theme/ResourceLoader.java
index 6f667d68901..7bb7e40a6d2 100644
--- a/services/src/main/java/org/keycloak/theme/ResourceLoader.java
+++ b/services/src/main/java/org/keycloak/theme/ResourceLoader.java
@@ -15,7 +15,12 @@ public class ResourceLoader {
         Path rootPath = Path.of("/", root).normalize().toAbsolutePath();
         Path resourcePath = rootPath.resolve(resource).normalize().toAbsolutePath();
         if (resourcePath.startsWith(rootPath)) {
-            URL url = classLoader().getResource(resourcePath.toString().substring(1));
+            if (File.separatorChar == '/') {
+                resource = resourcePath.toString().substring(1);
+            } else {
+                resource = resourcePath.toString().substring(2).replace('\\', '/');
+            }
+            URL url = classLoader().getResource(resource);
             return url != null ? url.openStream() : null;
         } else {
             return null;
diff --git a/testsuite/integration-arquillian/tests/base/testsuites/jdk-suite b/testsuite/integration-arquillian/tests/base/testsuites/jdk-suite
index adc953a1279..9b7150e99b1 100644
--- a/testsuite/integration-arquillian/tests/base/testsuites/jdk-suite
+++ b/testsuite/integration-arquillian/tests/base/testsuites/jdk-suite
@@ -15,3 +15,4 @@ SSOTest
 SamlClientTest
 TransactionsTest
 X509BrowserLoginTest
+ThemeResourceProviderTest