Improve hover text for reactions in Dark mode

Right now is the text shown when hovering over a reaction on an Issue/PR/commit comment/… white on a light-grey background.
While a contrast is there is it still a bit difficult to read the text, so the grey should perhaps be made slightly darker or the text should have a different color to make it stand out a bit better.

Here’s an image of what I mean (Name censored as it wasn’t me reacting):