How to set the content width?

Dear GeneratePress team,

How do you set the content width? My goal is to have all content max. 1140 pixels wide without any sidebars. Therefore I've tried the following:

1. I've created a custom template (a copy of the template from your site) and replaced some colors. No layout changes though.
2. In Customizer -> Layout -> Container: Container Width was set to 1140, Content Layout is set to Separate Containers, and Container Alignment is set to Boxes.
3. In Customizer -> Layout -> Sidebars: All (three) options are set to "Content (no sidebars)"
4. At Appearance -> Elements: One Layout element was created. Sidebars are set to "Content (no sidebars)", Footer: Disabled, Disabled, Elements: Secondary Navigation and Featured Image, Content: Content Areas is set to Default and Content Width is set to 1140. Display Rules: Location is set to Entire Site.

So far the result is that the content is shown in the same width as if the the sidebars are still enabled. I.e. the content is too narrow. Do you have any ideas what could be wrong here? Or how to fix it?

Thanks for your advise,
Michael.