inject_css.java

    webView.setWebViewClient(new WebViewClient() {

        @Override
        public void onPageFinished(WebView view, String url) {
            injectCSS();
            injectJS();
            super.onPageFinished(view, url);
        }
    });

    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';" +
                    "style.innerHTML = window.atob('" + encoded + "');" +
                    "parent.appendChild(style)" +
                    "})()");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void injectJS() {
        try {
            InputStream inputStream = getAssets().open("jscript.js");
            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 script = document.createElement('script');" +
                    "script.type = 'text/javascript';" +
                    "script.innerHTML = window.atob('" + encoded + "');" +
                    "parent.appendChild(script)" +
                    "})()");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }