All Privoxy built-in pages, i.e. error pages such as the "404 - No Such Domain" error page, the "BLOCKED" page and all pages of its web-based user interface, are generated from templates. (Privoxy must be running for the above links to work as intended.)
The templates are basically normal HTML files, but with place-holders (called symbols or exports), which Privoxy fills at run time. It is possible to edit the templates with a normal text editor, should you want to customize them. (Not recommended for the casual user). Should you create your own custom templates, you should use the config setting templdir to specify an alternate location, so your templates do not get overwritten during upgrades.
Note that just like in configuration files, lines starting with # are ignored when the templates are filled in.
The place-holders are of the form @name@, and you will find a list of available symbols, which vary from template to template, in the comments at the start of each file. Note that these comments are not always accurate, and that it's probably best to look at the existing HTML code to find out which symbols are supported and what they are filled in with.
A special application of this substitution mechanism is to make whole blocks of HTML code disappear when a specific symbol is set. We use this for many purposes, one of them being to include the beta warning in all our user interface (CGI) pages when Privoxy is in an alpha or beta development stage:
<!-- @if-unstable-start --> ... beta warning HTML code goes here ... <!-- if-unstable-end@ -->
If the "unstable" symbol is set, everything in between and including @if-unstable-start and if-unstable-end@ will disappear, leaving nothing but an empty comment:
There's also an if-then-else construct and an #include mechanism, but you'll sure find out if you are inclined to edit the templates ;-)
All templates refer to a style located at http://config.privoxy.org/send-stylesheet. This is, of course, locally served by Privoxy and the source for it can be found and edited in the cgi-style.css template.