Try to fix dark theme path

src/main/java/org/jabref/gui/preview/PreviewViewer.java

@@ -1,5 +1,7 @@
 package org.jabref.gui.preview;
 
+import java.net.URL;
+import java.util.Base64;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.regex.Pattern;
@@ -27,6 +29,7 @@
 import org.jabref.logic.search.SearchQuery;
 import org.jabref.model.database.BibDatabaseContext;
 import org.jabref.model.entry.BibEntry;
+import org.jabref.model.strings.StringUtil;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -116,9 +119,14 @@ public PreviewViewer(BibDatabaseContext database, DialogService dialogService, S
 
     public void setTheme(String theme) {
         if (theme.equals(ThemeLoader.DARK_CSS)) {
-            previewView.getEngine().setUserStyleSheetLocation(JabRefFrame.class.getResource(ThemeLoader.DARK_CSS).toString());
+            // We need to load the css file manually, due to a bug in the jdk
+            // TODO: Remove this workaround as soon as https://github.com/openjdk/jfx/pull/22 is merged
+            URL url = JabRefFrame.class.getResource(ThemeLoader.DARK_CSS);
+            String dataUrl = "data:text/css;charset=utf-8;base64," +
+                    Base64.getEncoder().encodeToString(StringUtil.getResourceFileAsString(url).getBytes());
+
+            previewView.getEngine().setUserStyleSheetLocation(dataUrl);
         }
-
     }
 
     private void highlightSearchPattern() {

src/main/java/org/jabref/model/strings/StringUtil.java

@@ -1,5 +1,12 @@
 package org.jabref.model.strings;
 
+import java.io.BufferedInputStream;
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.net.URL;
+import java.net.URLConnection;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
@@ -9,6 +16,7 @@
 import java.util.StringTokenizer;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+import java.util.stream.Collectors;
 
 import org.jabref.architecture.ApacheCommonsLang3Allowed;
 
@@ -721,4 +729,19 @@ public static boolean containsIgnoreCase(String text, String searchString) {
     public static String substringBetween(String str, String open, String close) {
         return StringUtils.substringBetween(str, open, close);
     }
+
+    public static String getResourceFileAsString(URL resource) {
+        try {
+            URLConnection conn = resource.openConnection();
+            conn.connect();
+
+            try (InputStream inputStream = new BufferedInputStream(conn.getInputStream());
+                 InputStreamReader inputStreamReader = new InputStreamReader(inputStream);
+                 BufferedReader reader = new BufferedReader(inputStreamReader)) {
+                return reader.lines().collect(Collectors.joining(System.lineSeparator()));
+            }
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+    }
 }

src/test/java/org/jabref/model/strings/StringUtilTest.java

@@ -21,7 +21,7 @@ void StringUtilClassIsSmall() throws Exception {
         Path path = Paths.get("src", "main", "java", StringUtil.class.getName().replace('.', '/') + ".java");
         int lineCount = Files.readAllLines(path, StandardCharsets.UTF_8).size();
 
-        assertTrue(lineCount <= 725, "StringUtil increased in size to " + lineCount + ". "
+        assertTrue(lineCount <= 749, "StringUtil increased in size to " + lineCount + ". "
                 + "We try to keep this class as small as possible. "
                 + "Thus think twice if you add something to StringUtil.");
     }