MoinMoin has a theme plugin system that makes it easy to change the look and feel without touching the core code. You can customize colors by editing the stylesheets, or create a totally new user interface by writing a new theme plugin in Python. When you upgrade MoinMoin, your theme is not touched - it will work with the new version, or needs only small changes due to theme API changes.
The 'classic' theme is heavily based on CSS. So for easy changes (like changing colours or fonts, moving around stuff), you maybe only need to change the CSS (like the starshine theme does to get dark).
When you do a new theme, copy all images from classic to your img directory and then modify as needed. If you modify an image, do not forget to update the icon dict in the theme code - the image geometry is stored there for faster rendering.
The theme code is called by moinmoin with (more or less) abstract data in a dictionary usually called d. The code then uses items in that dictionary to generate HTML (also heavily relying on CSS). The new theme system in 1.3 avoid using the data dictionary when possible, and instead call different moin parts to get the needed data.
When you do a new theme, first clone your favorite theme, then try to edit the stylesheets to get what you want. Only if you can't reach your goal using CSS only, override some functions in your theme code.
All themes are sub classes of MoinMoin.theme.ThemeBase which define the primitive user interface elements, like logo, username, title, searchform, editbar, actions menu etc. You should reuse these parts unless you need to change their content or create new element that you can not build from existing parts. The theme API is not stable yet, and might change in the future.
To install a theme, unpack the archive and copy the resulting top-level directory (e.g., leftsibebar2) to the wikis htdocs directory, e.g. /usr/share/moin/mywiki/htdocs. You should end up with the following subdirectories: css/, img/.
Setting the default theme
Once you have created all the necessary theme components and saved them as indicated above, the theme can be applied to your wiki by using the theme_default option in wikiconfig.py (see HelpOnConfiguration).
Sharing your theme
If you made a nice theme (of course adhering to HTML and CSS standards), package it like described below and put it onto ThemeMarket. Please put it under GPL license, if possible.
# <THEMENAME>-<VERSION>.tar.gz (or .zip), containing: <THEMENAME>/<THEMENAME>.py <THEMENAME>/img/*.png <THEMENAME>/css/*.css
When you don't have to make a theme
as a wiki user, you can override theme CSS settings by putting your own user CSS URL into UserPreferences - this CSS is loaded after the theme CSS, so you can override all you want changed. Your CSS file must be in UTF-8 encoding (and as ASCII is a subset of that, it is ok, too).
page_header1, page_header2, page_footer1 and page_footer2 can now be callables and will be called with the "request" object as a single argument (note that you should accept any keyword arguments in order to be compatible to future changes).