Reduce the padding for #mw-content. Perhaps also the sidebars and #mw-content-block (in desktop-mid and desktop-large views). Particularly for mobile view, where it takes up a much more significant portion of the available width:
As mentioned in T173152: Timeless: Usability issues on mobile devices
Inefficient use of horizontal space. Better to have edge to edge ui and small padding on content on narrow form factors
and T165945: Timeless: Re-evaluate width cutoffs (#4255838)
maybe kill some of the silly extra padding
and T197037: Timeless has a bit larger padding on Special:Translate
whitespace with literally nothing in it (it’s about 1-2em on each side, but I think 0.5em would be far enough).