Add site-level overrides to themes/hugo-book/assets/_variables.scss
(imported after _defaults.scss, so plain assignments override the theme):
--font-size: 17px base font up from the theme's 16px (all rem
sizes scale off it)
$container-max-width: 100rem content cap up from 80rem, so a 1920px
display no longer leaves ~310px dead gutters
on each side
$menu-width: 18.75rem sidebar (category list + search box + search
results pane) up from the 10rem theme default
so search results are readable
$mobile-breakpoint recomputed from the new values (it is derived
in _defaults.scss, so the override must redo
the arithmetic or the mobile collapse breaks)