Improve display of websites not specifying background color

The `index.html` of the front-end specifies `#000` as background color,
however, it seems like various websites found 'in the wild' don't
specify their own background color and thus default to `transparent`. As
a result of this, they tend to become mostly unreadable when displayed
using `openWebsite` since they often assume that they get displayed on a
white background. Setting white as background color on the container
element of the `<iframe>` works around this issue.

front/dist/resources/style/style.css

@@ -352,6 +352,7 @@ body {
 #cowebsite {
     position: fixed;
     transition: transform 0.5s;
+    background-color: white;
 }
 #cowebsite.loading {
     background-color: gray;