You can use the openedIcon and closedIcon options to use html for the toggle buttons. You can for example use Hero icons.
$('#tree1').tree({
closedIcon: $(
`<svg xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke-width="1.5" stroke="currentColor"
class="size-4">
<path stroke-linecap="round" stroke-linejoin="round" d="M17.25 8.25 21 12m0 0-3.75 3.75M21 12H3" />
</svg>`
),
openedIcon: $(
`<svg xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke-width="1.5" stroke="currentColor"
class="size-4">
<path stroke-linecap="round" stroke-linejoin="round" d="M19.5 13.5 12 21m0 0-7.5-7.5M12 21V3" />
</svg>`
)
});