Hi, I’ve seen the same behaviour with my USB-Infra Red remote controller. Volume+ is working but Volume- is not. This limitation manifested itself after installation of Alpha4. Note that I never installed any other Alpha version of OSMC. Using OpenElec I have no problems with my IR remote controller.
Furthermore, the new OSMC theme is an enhanced version of Conq as on one of the Skin Settings screens you see the option to reset all Conq parameters. Note that I like this new theme very much!!