I'm not sure we're on the same page.
I've got the latest version that's built for Ubuntu 16.04 (Mint 18.1),
19.10.2-1, from this PPA: deb
http://ppa.launchpad.net/rvm/smplayer/ubuntu xenial main.
That's not the issue (don't think).
However, from that PPA the latest
skins is 2:15.2.0-1~xenial1. That may be OK if theme or skins package versions don't necessarily match the main app versions.
The latest themes version - same PPA is 2:18.6.0-1~xenial1.
Re: when select interface option "skinnable GUI," right clicking the control bar, "edit control bar" option disappears. Is that how it should work?
Only using the basic, mini or mpc GUI allows editing the control bar (add or move buttons).