Set background to the navigation of Getting Started column (tootsuite pr #6163)

app/javascript/flavours/glitch/styles/components/index.scss

@@ -1881,6 +1881,11 @@
   cursor: default;
 }
 
+.getting-started__wrapper,
+.getting_started {
+  background: $ui-base-color;
+}
+
 .getting-started__wrapper {
   position: relative;
   overflow-y: auto;