Update background under context menu separator (#192)

Improve background for (context) menu separators

Before a slightly lighter variant of `nord0` was used as background
color for separators in menus, including all context menus.
The `PopupMenu.background` key [1] now uses the same background color
like the (context) menus itself.

[1]: https://github.com/JetBrains/intellij-community/blob/idea/213.6461.79/platform/platform-resources/src/themes/metadata/JDK.themeMetadata.json#L726



Co-authored-by: Arctic Ice Studio <development@arcticicestudio.com>
Co-authored-by: Sven Greb <development@svengreb.de>

Closes GH-190
GH-192

src/nord.theme.json

@@ -235,6 +235,9 @@
         "activeBackground": "#434c5e"
       }
     },
+    "PopupMenu": {
+      "background": "#323846"
+    },
     "ProgressBar": {
       "background": "#88c0d0",
       "failedColor": "#bf616a",