UI: Fix hard to read text for drag disabled hints

In 499dbb626a, the background color of drag tooltips were changed so
text becomes more readable. But multiple people were touching the same
code, so the disabled hint tooltips didn't get the same tweak. They
would benefit from them even more, since the red text is even harder to
read on the transparent background than the regular, white text.

source/blender/windowmanager/intern/wm_dragdrop.c

@@ -725,10 +725,12 @@ static void wm_drop_operator_draw(const char *name, int x, int y)
 static void wm_drop_redalert_draw(const char *redalert_str, int x, int y)
 {
   const uiFontStyle *fstyle = UI_FSTYLE_WIDGET;
-  const float col_bg[4] = {0.0f, 0.0f, 0.0f, 0.2f};
-  float col_fg[4];
+  const bTheme *btheme = UI_GetTheme();
+  const uiWidgetColors *wcol = &btheme->tui.wcol_tooltip;
 
+  float col_fg[4], col_bg[4];
   UI_GetThemeColor4fv(TH_REDALERT, col_fg);
+  rgba_uchar_to_float(col_bg, wcol->inner);
 
   UI_fontstyle_draw_simple_backdrop(fstyle, x, y, redalert_str, col_fg, col_bg);
 }