The monotones have symmetric CIELAB lightness differences, so switching from dark to light mode retains the same perceived contrast in brightness between each value. Each mode is equally readable. The accent colors are based off specific colorwheel relations and subsequently translated to CIELAB to ensure perceptual uniformity in terms of lightness. The hues themselves, as with the monotone ab values, have been adjusted within a small range to achieve the most pleasing combination of colors.
I hope that doesn't turn too many other folks off from the page and colorscheme. It may sound like a pseudoscience blurb, particularly if you aren't a color geek, but I sure didn't intend it to. It's all technically relevant and (I thought ;) tightly descriptive.
Lab was actually a big part of the initial inspiration for the colorscheme. I work a lot in lab space (Lab is more correct, but cumbersome to type) and for anything related to actual human vision it is head and shoulders above other color models/spaces. It's also awkward if you think in RGB and notoriously poorly supported outside of Photoshop (though most OS color management systems that I'm aware of translate everything back into Lab or a Lab equivalent).
The color relations are also foundational to palette creation in traditional graphic design. They are super useful in creating a palette that feels harmonious and (subjectively) unified.
I'd be happy to go into more detail, but I tend to go on at length and, much like when I get into talking about code with people not interested in code, talking about color spaces with those not interested is a sure fire turn off :)
CIELAB might not be pseudo-science, but the claim that the application of CIELAB makes this a good scheme is, at best, dubious.
The bright red and pink don't blend with the other colors at all. Their jarring contrast (and red hue) could be adequate for an alert message, but not for innocent and frequent syntax elements.
Maybe there is a math model that captures all relevant aspects that make a good syntax highlighting color scheme. But this model clearly doesn't.
