X3 doesn't really work like this. In X3 you can already set the width of all modules
context, folders and gallery, either globally or on a per-page basis. This should be most modern and flexible, because often you want wide galleries but text limited width,
like this.
Therefore, the only difference that this WP theme does, is create a "grey" background, making the design look "boxed". I tried to add some custom CSS, and the "boxed" background style itself is not hard to achieve
[screenshot], but there will be problems with fixed topbar and other special features
[screenshot].
Since custom widths is already a feature in X3, we wish do not wish to complicate the design with boxed-background appearance, which doesn't provide any added functionality other than cosmetic.