Background text difficult to read
I've seen others make the same observation. The answers to the exercises are usually dimmed or greyed out, making them very very difficult to read. This problem exists on laptop, tablet and mobile phone versions.
Also, the sans serif typeface being used is difficult to read at times, especially for 0,O, l, 1, and ı (the Turkish undotted i). A serif typestyle would be much more legible.
I'm currrently studying Turkish, but have encountered this problem with other languages too.