[fix] simple theme: unset width for images on mobile layout
* fix fallout from 2fbedc4316b4e8a8bce8bb9506f59aa61e1a2666
-> similar to 14fb187548a58144c5cd13c9292fcc52f5a025c5
* change: set image height to 10rem
-> this is in my opinion more sensible size for mobile