Hi pascalbuz,
The case is, the theme functionality does not support changing posts and pages width.
However, if you have HTML and CSS knowledge, you can change the width of the posts and a pages, as well as other appearance styles, by making the necessary changes to the style.css file of your theme.
If you do not know how to do this, and you’d like us to help you, you can always request a paid customization of the theme, and provide all the details on our Support Forum. This way, we could develop the functionality that would match all of your requirements.
Regards,
BestWebSoft Support Team