Changing content width

Hi everyone,

i am using the mantra theme with one column and no sidebar.
I am looking for an option to change the content width to a lower value.
In the mantra theme settings i can only edit the width for content and sidebar, but the max width is set to 1220px and i havent found an option to change this.

Does anyone know where i can find this value, so i can change it?

littleImp