How to define different width of a Container for different Breakpoints?

Hi
I try to define different Width for a Container depending on the Device.
For Desktopmode I did set the Width to 75%.
When I’m in Mobiledevice Mode I change the Width of the Container to 100%.
But when I go back to Desktopmode it is set to 100% too.
How can I keep it to 75% and only change to 100% for Mobiledevices?

Kind regards
Alain