Improve default background of public profile header (#7556)

Looks better when no header image is set

app/javascript/styles/mastodon/accounts.scss

 .card {
-  background-color: lighten($ui-base-color, 4%);
+  background-color: $base-shadow-color;
   background-size: cover;
   background-position: center;
   border-radius: 4px 4px 0 0;