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
maybe kill some of the silly extra padding
whitespace with literally nothing in it (it’s about 1-2em on each side, but I think 0.5em would be far enough).