Hey, during 2h training session I felt what I can only describe as straining my pattern-matching part of the brain. I've observed that "my eyes" were slow matching against the crosses, especially green crosses. Could we try making the crosses thicker and the green color darker?

This is not a solution for low contrast issues, but it might still help a bit: were you aware that you can zoom in on the browser to make the puzzles much larger? At my screen resolution, the puzzles are quite small at the default zoom settings, so I usually have the weekday puzzles on at least 150% zoom.

In both Firefox and Chrome, this can be done with CTRL plus Mouse Wheel, or with CTRL plus the + or - key. This changes the zoom level, while making an icon appear in the address bar which allows you to reset it to 100% if you so desire.

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As a separate tip, the F11 key can put either of those browsers into fullscreen mode, hiding several UI elements and giving the screen more space for large puzzles; I use that for the monthly puzzles in the Standard website.

I found myself zooming as well.
I also found that "Hide background line when a cross is placed" is solving this issue for me, although it is a trade-off with much smaller issue of recognizing cell shape when there are few edges already crossed.