would be the same here if you turn off the right hand column..
since the style/theme uses a flex width scheme.. it is intended to fill the entire width and the author content is a percentage of that..
it's hardwired too
<td width="17%" valign="top" class="author-content"> to 17%
so it comes down to a philosophy dealio for you.. you can set your board width to a fixed value which is less and then the author-content will shrink..
additionally, you could actually modify the .tpl file to remove the hardwired width="17%" and rely on the class="author-content" instead..
this would allow you to change it in each style as desired..
-- personally --
i go with fixed width board themes styles and remove all 'percentage' based widths from installations.. but that is a board owner choice...
i have a 1920px width monitor and having a board stretch the entire screen looks 'bad' to me..
matter of fact, for THIS site, i modify the CSS in my browser to override the flex width to a fixed one..