There's not enough contrast. If we're limited to canned color schemes they should pass usability tests. I tested a page at
Check My Colours - Analyse the color contrast of your web pages and here are the results:
Testing done on 326 elements
Luminosity Contrast Ratio: 118 failures
Brightness difference: 112 failures
Color difference: 148 failures
Any online usability/accessibility page checker will report similar failures - there's just not enough contrast between link text and background.
The default color scheme should have zero contrast issues.