Improve dark mode style in Firefox (Options page) #2791

Our options page supports dark mode, but its colors are modeled after Chrome and not Firefox. You can see the awkward "different gray" background:

<img width="425" alt="" src="https://user-images.githubusercontent.com/1402241/74593763-cf6c7080-5069-11ea-9d00-83830732b413.png">

PR welcome to fix this elegantly