I can tell you where to look for thr CSS but not why it is a problem,. Especially from an image.

By default the CSS file for this is in the folder:

wp-content/sp-resources/forum-plugins/tinymce-richtext/resources/skins/SPlightgray/skin.min.css

and the rules you need to look at all start with the class name .mce-tooltip

OK, here is how it looks like. What would you suggest that I try change?

Well.. unless you have changed it it is going to look the same as mine and mine works OK. The problem you seem to have is probably a knock on effect from some other plugin or WP theme. I am really not sure if you can make changes to the editor skin to correct that. Might be wrong of course.