Webkit: support rendering dark controls

[nephele]

For webkit we currently render controls in "whatever the system has" This causes quite a bit of problems if the system is in a dark mode but the website is not, and vice versa! To fix this I want to implement support for the color-scheme in webkit, the only problem is that if the system is in light mode we don't know what the dark mode colors are, and for dark mode we don't know what the light mode colors are. I would trefore use the default colors in this case (perhaps the decorator can in the future provide those?)

Please comment on the aproach. example provided below

bool RenderThemeHaiku::paintSliderTrack(const RenderObject& object, const PaintInfo& info, const IntRect& intRect)
{
    rgb_color base = ui_color(B_PANEL_BACKGROUND_COLOR);
+    if (object->useDarkAppearence()) {
+        if (base.Brightness() < 127) // system is in dark mode
+            base = BPrivate::kDefaultColorsDark[B_PANEL_BACKGROUND_COLOR];
+    } else {
+        if (base.Brightness() < 127) // system is in dark mode
+            base = BPrivate::kDefaultColors[B_PANEL_BACKGROUND_COLOR];
+    }
    rgb_color background = base;
        // TODO: From PaintInfo?
    BRect rect = intRect;

And an example of a hard to see rendering with the current code