Ahhh, I think it is a monitor calibration issue then. On my computer, the highlighting in Calibre shows up just perfect. Not too dark so it is distracting but not too light that I can't see it easily.
Why don't you go here and check out your monitor's contrast? Look particularly at the last strip, the grayscale one, in the first section on the page. Make sure you can distinguish between every shade. If you can't, then your monitor needs a brightness and/or contrast adjustment. On the computer I am using right now, an Asus netbook, every shade/block looks distinct from the other.
http://www.lagom.nl/lcd-test/contrast.php