Improve dark mode test in wxAuiGenericTabArt

Use wxSystemSettings for this instead of manually checking the base
colour components which is both more verbose/difficult and less
reliable.

src/aui/tabart.cpp

@@ -447,9 +447,8 @@ void wxAuiGenericTabArt::DrawTab(wxDC& dc,
     int drawn_tab_yoff = border_points[1].y;
     int drawn_tab_height = border_points[0].y - border_points[1].y;
 
-    bool isdark = (m_baseColour.Red() < 75)
-        && (m_baseColour.Green() < 75)
-        && (m_baseColour.Blue() < 75);
+    bool isdark = wxSystemSettings::GetAppearance().IsUsingDarkBackground();
+
     wxColor back_color = m_baseColour;
     if (page.active)
     {