I am seeing this issue return in version 2020.2.513 with the default css. If I turn on selectable, when I select the range font color changes to white, which by itself is okay since the background of the cells changes to orange. When I select the range, the font and background return to normal. However, when I click in a cell to edit it, the font color changes to white which is now a problem because the background during editing is also white.
I was able to workaround this by changing the color attribute from #000 to #ffffff for the below css class