Hueman boxed layout remove the transparent border

Using Hueman theme in boxed layout, how could I remove the grey transparent border in left and right ?

I’ve already searched the web thoroughly, but nothing worked.
Tried this custom CSS too,

.boxed #wrapper {
    box-shadow: none;
}

From this forum post , https://www.ads-software.com/support/topic/how-to-remove-border-from-boxed-layout

But it didn’t worked for me, please help, and thanks in advance.