Add initial bg color to panels (#4268)

src/panels/custom/ha-panel-custom.ts

@@ -129,6 +129,7 @@ export class HaPanelCustom extends UpdatingElement {
         width: 100%;
         height: 100%;
         display: block;
+        background-color: var(--primary-background-color);
       }
     </style>
     <iframe></iframe>

src/panels/iframe/ha-panel-iframe.js

@@ -13,6 +13,7 @@ class HaPanelIframe extends PolymerElement {
           border: 0;
           width: 100%;
           height: calc(100% - 64px);
+          background-color: var(--primary-background-color);
         }
       </style>
       <app-toolbar>