Merge branch '382-focus-hardly-visible-in-fullscreen' into 'master'

Inset for fullscreen buttons to highlight focus

Closes #382

See merge request arsnova/frag.jetzt!391

src/app/app.component.scss

@@ -58,7 +58,11 @@ main {
   display:none;
   box-shadow:0px 2px 2px rgba(0,0,0,0.2);
   transition:all 0.2s ease-in-out;
-}
+  >button:focus{
+    box-shadow:inset 0 0 0 1px var(--on-surface);
+    }
+  }
+
 
 .rescale_overlay>button{
   margin-top:5px;