LPS-51699 Cover Image for Blogs

cc/ @ipeychev @sergiogonzalez 
From what I can tell, I *believe* this is a hack to only load this CSS inside of firefox. However, we have more clear ways of indicating that.
Create a new block outside of the `.portlet-blogs` section, like so:
```
.firefox .portlet-blogs fieldset.input-container {
    display: table-column;
}
```

This makes it a bit easier to see that it's trying to target a specific browser (or, if you're trying to target any gecko based browser, you can replace `.firefox` with `.gecko`)