The styling for the FNCTabset has the colour of the selected tab set incorrectly. E.g. using the SlateGray style with the treeview and listbox the selected items are green as expected. For the tabset the selected tab is dark with the background tabs being green. This leads to a very confusing UI.
The appearance is by design. The tab is actually part of the page, which is black. When we would change the colors, the page color would be the same as the selected color, which would be green, therefore we have chosen this style. Even TPageControl & TTabSet is not consistent. TTabSet has full colored selected tabs, while TPageControl has a color line under the tab.
You can change the behaviour if you want. By switching the colors after setting AdaptToStyle := True