inject CSS to a site with webview in android

I was able to inject css by using "evaluateJavascript" which was added to WebView in API 19 https://developer.android.com/reference/android/webkit/WebView

Example in Kotlin:

private lateinit var webView: WebView

override fun onCreate(savedInstanceState: Bundle?) {

    super.onCreate(savedInstanceState)

    //...

    webView = findViewById(R.id.your_webview_name)

    webView.settings.javaScriptEnabled = true

    webView.webViewClient = object : WebViewClient() {

        override fun onPageFinished(view: WebView, url: String) {

            val css = ".menu_height{height:35px;}.. etc..." //your css as String
            val js = "var style = document.createElement('style'); style.innerHTML = '$css'; document.head.appendChild(style);"
            webView.evaluateJavascript(js,null)
            super.onPageFinished(view, url)
        }
    }

    webView.loadUrl("https://mywepage.com") //webpage you want to load   
}

UPDATE: The code above had issues applying all of the injected CSS. After consulting with my web developer, we decided to inject the link to the CSS file instead of the CSS code itself. I changed the values of the css and js variables ->

val css = "https://mywebsite.com/css/custom_app_styles.css"
val js = "var link = document.createElement('link'); link.setAttribute('href','$css'); link.setAttribute('rel', 'stylesheet'); link.setAttribute('type','text/css'); document.head.appendChild(link);"

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);
  }
}