If you can't find a theme that does what you need it to, you will have to create your own: https://docs.moodle.org/dev/Themes
The app and the website use completely separate CSS frameworks (ionic for mobile, Bootstrap for website) which means you wont be able to re-use Moodle app CSS for a Moodle website theme.
As for the "Moodle app additional features", it depends exactly what features you mean. Some of these are coded specifically into the app, but some have an equivalent on the web. The bottom line is that displaying a Moodle web page in a webview component isn't going to let you embed a full Moodle app experience in your app. You'd either have to use the Moodle app itself, or re-implement the features you want in your app using web service calls.