data/meld.css, gutterrendererchunk: Try to adapt to new themeing colors

Behaviour of several widgets and the meaning of base colours appears to
have changed in 3.20. This commit brings the LinkMap and gutter
renderers in line to both use the theme base background, though this
does look... fairly dated.

Author: kaiw

data/meld.css

@@ -1,7 +1,7 @@
 
-.meld-notebook-toolbar.toolbar {
+.meld-notebook-toolbar {
 	background-image: none;
-    background-color: @theme_base_color;
+    background-color: @theme_bg_color;
     border-width: 0 0 1px 0;
     border-style: solid;
     border-color: @borders;
@@ -14,7 +14,7 @@ link-map {
     border-width: 0 0 1px 0;
     border-style: solid;
     border-color: @borders;
-    background-color: mix(@theme_bg_color, @theme_base_color, 0.5);
+    background-color: @theme_bg_color;
 }
 
 .meld-vc-console-pane {

meld/gutterrendererchunk.py

@@ -44,12 +44,23 @@ def on_setting_changed(self, meldsettings, key):
 
     def draw_chunks(
             self, context, background_area, cell_area, start, end, state):
+
+        stylecontext = self.props.view.get_style_context()
+        background_set, background_rgba = (
+            stylecontext.lookup_color('theme_bg_color'))
+
         line = start.get_line()
         chunk_index = self.linediffer.locate_chunk(self.from_pane, line)[0]
 
         context.save()
         context.set_line_width(1.0)
 
+        context.rectangle(
+            background_area.x, background_area.y,
+            background_area.width, background_area.height)
+        context.set_source_rgba(*background_rgba)
+        context.fill()
+
         if chunk_index is not None:
             chunk = self.linediffer.get_chunk(
                 chunk_index, self.from_pane, self.to_pane)