Set the GTK background color to white.

anntzer · anntzer · commit dfd444ca4da7 · 2021-01-31T18:54:43.000+01:00
This is consistent with the other backends (although this seems to be
broken on wx too), which allows semi-transparent figures to be rendered
against a consistent background (this can be checked e.g. with
`figure(facecolor=(0, 0, 0, 0))` or `figure(facecolor=(1, 1, 1, .5))`),
which would previously be rendered differently on gtk3 when using a dark
theme.

While override_background_color is marked as deprecated on gtk3, the
alternative seems to involve a bunch of CSS, and it seems unlikely to
ever be removed now that gtk development has switched to gtk4...
diff --git a/lib/matplotlib/backends/backend_gtk3.py b/lib/matplotlib/backends/backend_gtk3.py
@@ -126,6 +126,8 @@ def __init__(self, figure):
 
         self.set_double_buffered(True)
         self.set_can_focus(True)
+        self.override_background_color(
+            Gtk.StateFlags.NORMAL, Gdk.RGBA(1, 1, 1, 1))
 
         renderer_init = _api.deprecate_method_override(
             __class__._renderer_init, self, allow_empty=True, since="3.3",
