Typically a contrast ratio of at least 4.5:1 between the 'relative luminance' of text and its background colour is recommended for normal text. However, that's oversimplified and doesn't address everything. The text in that image [Post #1] does look smaller than a normal: 16px or 12pt.
Perhaps on a small handheld device, with a poorly lit screen, that text colour might not be as obvious to distinguish either. If you are not affected by Colour vision deficiency, and are using a desktop monitor, you shouldn't have an issue distinguishing between those colours.
Also as was mentioned the background is transparent with some opacity. Therefore, the background is not always single solid colour, which could reduce text legibility.
I don't have Colour vision deficiency. Albeit about 1 in 12 men and 1 in 200 women have colour vision deficiency.