Huge thanks @matthewsjones! Your code is extremely helpful!
I’ve adjusted it a bit to allow specifying the page width in percent:
def _max_width_(prcnt_width:int = 75):
    max_width_str = f"max-width: {prcnt_width}%;"
    st.markdown(f""" 
                <style> 
                .reportview-container .main .block-container{{{max_width_str}}}
                </style>    
                """, 
                unsafe_allow_html=True,
    )