Since 6.14 version JxBrowser provides API that allows injecting custom style sheet (CSS) into every web page loaded in the Browser instance. The following example demonstrates how to use this functionality:

import com.teamdev.jxbrowser.chromium.Browser;
import com.teamdev.jxbrowser.chromium.swing.BrowserView;

import javax.swing.*;
import java.awt.*;

public class CustomCSS {
    public static void main(String[] args) {
        Browser browser = new Browser();
        BrowserView view = new BrowserView(browser);

        JFrame frame = new JFrame();
        frame.add(view, BorderLayout.CENTER);
        frame.setSize(800, 600);

        browser.setCustomStyleSheet("body { background-color: orange; }");

Note: the injected CSS won't override already defined CSS properties on the loaded web page.

To reset injected CSS call the browser.setCustomStyleSheet("") method.