"Plus" button on tab bar is nearly invisible
I am using "Opera 27.0.1689.54" on "Windows 8.1 Update 1" with "Windows Classic Visual Style for Windows 8.1/Update 1" from "http://xxinightxx.deviantart.com/art/Windows-Classic-Vs-8-1-Update-1-457573957". In Windows' "Control Panel/Personalization/Color and Appearance", I changed the color of the active window border to very dark blue. (This is the color of the active window border of any Windows program which is active. The text in the top border of an active window is white.)
Now in Opera, when the Opera window is the active window, the Opera tab bar has the same very dark blue color as the active window border. Because of this very dark blue color of the Opera tab bar, the "plus" button on the tab bar (for opening a new tab), is nearly invisible (because of the lack of contrast between the "plus" and its background). I would expect a light "plus" when the background is dark, but it is a black "plus".
How can I make this "plus" better visible (i.e. much lighter, or white), without having to change the color of the active window border?