More generally, it would be nice to have a system of theme/skin (as jQuery UI has for instance) to customize the appearance of the chessboards.

This is quite a lot of work, so I will not doing it right now. However, I could add CSS classes for light and dark squares quite rapidly, so that you will be able change the color of board squares with a piece of CSS code. I will try to do it in the next days (before the end of June).

Thank you for such comprehensive answer. It's true that my previous question was a bit unclear. I wrote comment on Github yesterday explaining my point of view, but accidentally form was deleted, so in short:

1. Chess font in notation is great improvement and I think Alpha font is really good. Moroever you can change it by CSS so it's even better.
2. I was thinking about including chess font (instead of bitmaps) in chessboard (diagram), but i'm not sure if it is possible / worth of doing (i'm not so good in javascript). I thought it would be better for performance (loading speed) but i'm not sure. Also it would be possible to change the color of pieces easily (eg. for dark brown and light brown instead of black and white).
   2a) If changing bitmaps to fonts won't be included in next verisons (or won't be included in near future) I think it will be better for me (and everybody who want change the look of chess pieces) just to prepare own sprite file with bitmaps. IMHO recent pieces bitmaps are not bad, but needs a small "lifting" - smooth edges, maybe smaller size. Please tell me what do you think and what are your plans.
3. As you said earlier long-term solution for flexibility and customizations would be to include options in wordpress admin panel (eg. changing colors, chess fonts, sizes etc). but I have full understanding that this is time-consuming. You do a really good job anyway pushing this script forward so quickly.

To answer your second point, I would say that, in my opinion, changing bitmaps for fonts in diagrams is not a good idea:

• the performance gain is rather hypothetic (depending on how web browsers manage fonts, it even may be slower),
• this might induce some (maybe browser-dependent) issues to ensure that the chess pieces have the appropriate size, and are vertically and horizontally aligned with the center of the square.
  So I think this modification will not be included in future versions.

However, I agree that it would be useful to allow chess piece customization in the diagrams. I think that, in future versions, several sets of sprite files will be included with the plugin (please feel free to suggest some).

Version 2.99.1 (just released) provides CSS classes for dark- and light-squares.

In version 4.3, it is possible to change the default color/piece themes. However, alternative themes must be chosen among the ones included with the plugin: users cannot build their own color/piece themes. This feature will be implemented in a future version.

As of version 4.5, users can create their own color/piece themes.