Consider a dropshadow for the MR popover to increase contrast

It's hard to spot the popover when using GitLab website in 90% zoom (I know this is not intended, but it's what work for me). Also even on 100% zoom, I find it difficult to get the right contrast between what is the new white popover versus the rest of the content.

On the previous popovers that we still use (the ones with black background) this is not an issue.

So here is the proposal:

.popover {
    box-shadow: 2px 2px 10px 1px lightgray;
}