In Options | Settings-theme-ui..., I am unable to find an attribute that changes the colour of the sidebar-selected-item-background (but I know it can be changed, as some themes do change it).
Thanks. That did change it--but not until I saved the theme and then moused over the selected buttons in the sidebar. Speaking of which, I see no change in any buttons (sidebar, find/replace window) when mousing over them (colour, border, etc.), although the tooltips do appear.
The theme-item which I mentioned, is used to show checked buttons. the mouse-over is another theme-item! look at "buttons, BG, mouse-over" and all "buttons, ******" items.
Yes, I have tried all the "buttons, BG,..." items, including mouse-over, but changing them doesn't actually affect mousing over. The item you mentioned previously, did change the checked buttons background... but not until I moused over them!
Windows 10. Most changes I make in the theme settings dialog do appear when I press Apply, just not that background. However, there are never any changes on mouse-over, in any of the themes that I've tried.