The change of background colors is a deliberate design choice. The content is long and the background helps the reader, especially on mobile, to understand in which section he currently is.

It's a presentation in the form of an article. Same content, different medium.

Ya the switch to white font on dark bg is no good.

Here's a fix:

   $('.blue-grey').css("background-color","#F0F0F0").css("color","#0F0F0F")