I would suggest to work on responsiveness, layout is broken for any width between 400px and 1200px.
Also header is not 100% width on screens with width > 1400px.
More interactivity would be good too:
animation for region filter dropdown on open/close
cursor: pointer; for country cards
animation for theme switch
input is frozen when typing some characters - it should not freeze. Seems like a debounce but then you need something like 2 states - one for input value itself and the second one is search value. Or uncontrolled mode with debounced reaction.
region filter is frozen after item selection
On the country details page:
it would be nice to hide "border countries" title at all if there are no border countries (such as Northern Mariana Islands, Bermuda, etc.)
interactivity for Back button (scale it or change bg on hover)