When you reveal a spoiler the black rectangles go away and it then renders the text in white, but on a white background (light coloured theme) The text renders is exactly the same colour as the background and so you can not see it unless you highlight the post, this renders the background of the post as light grey and there is just about enough contrast difference to now read the text.

When spoiler text is revealed it should render in the same colour as normal text. I'm not sure why it's white.

See images below for demonstration.