Set the GTK background color to white.

anntzer · anntzer · commit 3c832377fb4c · 2021-02-10T13:39:58.000+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.
diff --git a/lib/matplotlib/backends/backend_gtk3.py b/lib/matplotlib/backends/backend_gtk3.py
@@ -127,6 +127,12 @@ def __init__(self, figure=None):
 
         self.set_can_focus(True)
 
+        css = Gtk.CssProvider()
+        css.load_from_data(b".matplotlib-canvas { background-color: white; }")
+        style_ctx = self.get_style_context()
+        style_ctx.add_provider(css, Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION)
+        style_ctx.add_class("matplotlib-canvas")
+
         renderer_init = _api.deprecate_method_override(
             __class__._renderer_init, self, allow_empty=True, since="3.3",
             addendum="Please initialize the renderer, if needed, in the "
