Fix: close button in hover tooltips overlap text
Created by: attfarhan
Fixes https://github.com/sourcegraph/sourcegraph/issues/1935.
Rather than adding padding as suggested in the issue, which would force us to make the close button use non-absolute positioning to affect any change, I just made the background for the close button match the background of the card. This makes it not look broken when it collides with text. @francisschmaltz I think this is a good fix, lmk if you think otherwise/have a better suggestion.