inject CSS to a site with webview in android

You can’t inject CSS directly however you can use Javascript to manipulate page dom.

public class MainActivity extends ActionBarActivity {

  WebView webView;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    webView = new WebView(this);
    setContentView(webView);

    // Enable Javascript
    webView.getSettings().setJavaScriptEnabled(true);

    // Add a WebViewClient
    webView.setWebViewClient(new WebViewClient() {

        @Override
        public void onPageFinished(WebView view, String url) {

            // Inject CSS when page is done loading
            injectCSS();
            super.onPageFinished(view, url);
        }
    });

    // Load a webpage
    webView.loadUrl("https://www.google.com");
}

// Inject CSS method: read style.css from assets folder
// Append stylesheet to document head
private void injectCSS() {
    try {
        InputStream inputStream = getAssets().open("style.css");
        byte[] buffer = new byte[inputStream.available()];
        inputStream.read(buffer);
        inputStream.close();
        String encoded = Base64.encodeToString(buffer, Base64.NO_WRAP);
        webView.loadUrl("javascript:(function() {" +
                "var parent = document.getElementsByTagName('head').item(0);" +
                "var style = document.createElement('style');" +
                "style.type="text/css";" +
                // Tell the browser to BASE64-decode the string into your script !!!
                "style.innerHTML = window.atob('" + encoded + "');" +
                "parent.appendChild(style)" +
                "})()");
    } catch (Exception e) {
        e.printStackTrace();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
  }
}