[se-edu#415] Ui: fix default PersonListPanel background color (se-edu…

…#791)

Normally, the background of the PersonListPanel is gray in color.
However, it changes to white color when no Person is listed.

This does not fit the dark theme style of other parts of the Ui as the
difference in color makes the PersonListPanel stand out rather
prominently.

Let's fix this by setting the default background color of
PersonListPanel to match the rest of the Ui.

src/main/resources/view/DarkTheme.css

@@ -90,6 +90,7 @@
 .list-view {
     -fx-background-insets: 0;
     -fx-padding: 0;
+    -fx-background-color: derive(#1d1d1d, 20%);
 }
 
 .list-cell {