I'm using Opera on a Chromebook tablet capable of running Android apps.
After hunting all around I simply cannot find where to customize what appears in the sidebar. Settings, Profile, Customizstion, all do not seem to offer that option.
Thanks @leocg, but there sure is a sidebar, a narrow one that expands when the hamburger icon at the top is clicked. And on a tablet, a "right-click" is typically a long press. Nothing happens there.