customization of source-highlight is difficult or impossible

Description: Ubuntu 22.04.2 LTS
Release: 22.04

$ apt show source-highlight
Package: source-highlight
Version: 3.1.9-4.1build2

Source-highlight is known not to support dark-theme terminals well with the default settings (in particular the 'default' esc.style is particularly bad).

It seems there are only two ways of customizing source-highlight

1) Edit the contents of /usr/share/source-highlight
2) Make a copy of the entire /usr/share/source-highlight directory and customize that, then use source-highlight-config to change where the package picks up the configuration files.

The first approach means modifying the distribution files directly, which makes upgrading the package difficult.

The second approach means that after an upgrade all the copied files need to be updated and users might miss this.

The configuration system should really use a search path for its configuration files, and files like lang.map should be made to accumulate map information from multiple locations. This would make it far easier to customize colors for specific packages or environments and to add local custom color rules.