Bug 1696988 - Add backgrounds for non-native tooltips on Linux and OSX. r=ntim

The Linux styles match Adwaita trunk[1], so it's a bit smaller than the
native ones but I think that's fine (it matches other themes more
closely, like Arc).

On OSX the system color needs to be tweaked (will do in a follow-up)
since it is windows-style yellow right now, which is rather ugly. But
this is a progression regardless.

[1]: https://gitlab.gnome.org/GNOME/gtk/-/issues/3352

Differential Revision: https://phabricator.services.mozilla.com/D107548

toolkit/themes/linux/global/tooltip.css

@@ -13,7 +13,11 @@ tooltip {
   appearance: auto;
   -moz-default-appearance: tooltip;
   max-width: 40em;
+
+  padding: 6px 10px; /* Matches Adwaita. */
+
   color: InfoText;
+  background-color: InfoBackground;
   font: message-box;
 }
 

toolkit/themes/osx/global/tooltip.css

@@ -16,6 +16,7 @@ tooltip {
   padding: 2px 3px;
   max-width: 40em;
   color: InfoText;
+  background-color: InfoBackground;
   font: message-box;
   cursor: default;
 }

widget/gtk/nsNativeThemeGTK.cpp

@@ -1377,6 +1377,7 @@ bool nsNativeThemeGTK::GetWidgetPadding(nsDeviceContext* aContext,
   switch (aAppearance) {
     case StyleAppearance::ButtonFocus:
     case StyleAppearance::Toolbarbutton:
+    case StyleAppearance::Tooltip:
     case StyleAppearance::MozWindowButtonBox:
     case StyleAppearance::MozWindowButtonClose:
     case StyleAppearance::MozWindowButtonMinimize: