I see a few things as useful to customize a forum without needing to fork a theme:
Being able to identify the website in the bookmarks or trough the tab open is useful and essential. So far all forum software I've seen requires either forking or hacking the theme to implement this. If it is possible to set a custom favicon.ico file somewhere and make the software use that instead of the theme's default, it'd a major advantage to the general forum software out there and a huge recognizability leap.
The same applies to the logo on the header, being able to hide it, replace it, or even make the logo be the title instead (by hiding the forum's title in the image's alt attribute), that would avoid many of the horrible hacks I've seen so far around the web. (on one site I used to work at, I remember someone replaced the forum's title string with blank, so that the logo could replace it, the result, empty <title> string, empty e-mails, site unrecognizable on browsers with no image support or such disabled, etc. All because their logo integrated the title in itself and they didn't want to display the title in that situation)
• Color Scheme
I know this is already planned, but having a few custom color schemes would be good. Another idea would be to also allow override stylesheets, and provide a simple one with instructions on which values to change to make a new color scheme.
This, together with your about.html, I think are the absolute essential. Perhaps besides that also a posting-rules.html, to add or replace the text next to the new-thread/reply box with custom links to forum rules, tips,etc for specific forums.
Jose Pedro Arvela added on
If a favicon.ico is found on the settings folder (assuming there's a settings folder), the use it instead of the theme's favicon.
A custom logo is used if a logo.png or logo-replace.png exists, with logo.png being used in the same way it is used by default and logo-replace.png used to replace the header text with the logo. If both exist, logo.png should be used due to the better markup and lesser problems on text-only browsers.
• Color Scheme
If a custom-style.css exists, append it after the default theme's style like so:
% HTML <link rel="stylesheet" href="/themes/greyscale/theme.css" />
<link rel="stylesheet" href="custom-style.css" /> %
For people to relate it with color schemes, we should provide a custom blank CSS file with only the colors for people to change.
Kroc added on
(the code block bug was due to me forgetting to upload the latest NNF to camen design; I’ve retroactively fixed your post for clarity)