Since there was some negative commentary about the looks of Knights in my previous post, I took the popular advice and used Plasma’s clock in Knights. And you were correct: it looks much more pleasing to the eye now. The Plasma guys know what they’re doing.
And here is a shot with the KDE default widget style and colors. The theme is a variation of the previous one, again from the XBoard program. It doesn’t really fit in either, I know.
This unfortunately introduces a new dependency: You need at least one Plasma theme, and those reside in kdebase-runtime-data. This should not be needed for a chess game, so I plan to add a check for a theme and fall back to the previous solution if none is found. As noted by Aaron in the comments, hard-depending on kdebase-runtime is fine. Thanks for clearing that up, and sorry for spreading FUD.
The second thing was the horrible white background. The easy solution is to use a dark theme, all programs looks better that way. Really, be sure to try it if you haven’t already. Unfortunately, this is not so good on laptops (at least not on mine) because it does not mix well with lower screen brightness. Anyway, the proper way to do it is to allow the theme to specify its own background. I’m no artist, so no themes have it yet, but adding a simple mono-color rectangle that matches the board colors can make a big difference. I did check it though, with an even more horrible blue rectangle, so I’m sure it works. You just don’t want to see my tests, believe me.
The third thing is a kde-styled theme. There are six themes available now, but most are copies from other program and none of them looks kde-ish. I have wrote to kde-artists, but got little response (only about using the Plasma clock, big thanks for that). So if anyone is interested in making a nice kde-theme set of chess pieces, you’re very welcome 
