Nice! Consider this your rare opportunity to talk about your karma without repercussion. Don't make it a habit, though; I'm sure you have better things to talk about anyways ;)
Think of it like a video game, which often gives people surprises by unlocking the hidden feature based on unclear conditions. I was really impressed was I discovered it for the first time. This feature is more of a Easter egg than a practical option.
If you really NEED to change the color, you can simply load a custom CSS in your web browser.
It would be awesome to be able to see the up and down vote counts on comments. If you think showing that might adversely affect HN, you could make that super exclusive, like requiring over 10k in karma ;)
Enhance/increase the font size, the size of the voting buttons, the gap between the voting buttons, and the size of the collapse buttons for users who have more than 50 karma (they must have perseverance and patience to stick around, so why not reward them)? I really find the usability of these very difficult and hit the wrong one many a times. Simplicity is nice, but going through HN comments and acting on them is painful, more so on touchscreen devices.
I think those of us in top 5 auto-stigmatize about that fact enough without needing an explicit caste system, which are _impossible to make meaningless_.
Why on earth is there a karma requirement for this, let alone such a high one?