It’s not contrast, it’s saturation. The text is pure cyan, which has maximum saturation in the RGB space. When you use highly saturated colours, edges can tend to ‘bleed’ a bit, especially if your monitor isn’t great or your vision isn’t perfect – so they’re ok for display text but not body text (the headings suffer less from this issue – though the colour still grates somewhat against that of the background somewhat; and, less objectively, pure cyan is minging).
It’s a bit like trying to read text that’s written entirely in neon lights – the visual equivalent of eating a five-course meal that’s been made entirely from Haribo Tangfastics.
White or near-white text is much more easy on the eye with that background.
