doc: _static: css: fix navtree +/- icon margins

Fixed selectors that probably went stale overtime since the custom.css
was adopted from Godot and RTD having evolved in the meantime and that
were causing +/- icons to expand/collapse the navtree to be putting
margins where they shouldn't have.

Fixes zephyrproject-rtos/zephyr#78974

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>