Different menu site on mobile

Hi guys,
I am a total newbee so sorry, if I use the wrong words 😉
I’m using the theme simpl. The theme is mobile responsive and everything on the mobile looks more or less fine. BUT I want to have a onepager on the mobile version of course with a different kind of menu, so I don’t jump around between pages.
I created already a ‘onepager’ page and the right menu and both works if I use it in the desktop version as well as in the mobile. But how can I have a different setup for mobile slash desktop.
Is there a possibility to ‘just’ add a code in the custom css within the theme?
Thanks a lot for the help
Andy