Add this piece of configuration into ~/.config/openbox/lxde-rc.xml. Then, Fn+Home and Fn+End can increase and decrease screen's brightness individually. After saving, issue command ``$ openbox --reconfigure`` to load new configuration.

Adjust screen bightness in LXDE

<keybind key="XF86MonBrightnessUp">
  <action name="Execute">
    <command>xbacklight -inc 10%</command>
  </action>
</keybind>
<keybind key="XF86MonBrightnessDown">
  <action name="Execute">
    <command>xbacklight -dec 10%</command>
  </action>
</keybind>