"Use subreddit style" toggle - don't show on top of menus/popups

Ensure that the "Use subreddit styling" toggle doesn't overstep its bounds by temporarily disabling the  [new and improved z-index](https://github.com/honestbleeps/Reddit-Enhancement-Suite/commit/9f70f63e84275a41b4a6ba3e1b44cdc8db5500d0) when RES pop-up/drop-down elements are showing.