Add border around sim in high contrast (#1161)

* Add border around sim in high contrast

* Specify border width

sim/visuals/microbit.ts

@@ -162,6 +162,9 @@ namespace pxsim.visuals {
         }
     `;
     const MB_HIGHCONTRAST = `
+svg.sim {
+    border: solid white 0.15em;
+}
 .sim-led {
     stroke: red;
 }