style: center landing icons (openscd#529)

Make sure to center the landing icons on the page

Author: ca-d

src/open-scd.ts

@@ -135,9 +135,11 @@ export class OpenSCD extends Hosting(
 
     .landing {
       position: absolute;
+      text-align: center;
       top: 50%;
       left: 50%;
       transform: translate(-50%, -50%);
+      width: 100%;
     }
 
     .landing_icon:hover {