To get custom highlighting for a simple "Todo" style list in the gnome application gEdit
Step 1) Added a new language definition in the directory /usr/share/gtksourceview-2.0/language-specs
todo.lang:
<?xml version="1.0" encoding="UTF-8"?>
<language id="todo" _name="TODO" version="2.0" _section="Others">
<metadata>
<property name="mimetypes">text/plain</property>
<property name="globs">*.txt</property>
</metadata>
<styles>
<style id="done" _name="Done" map-to="def:done"/>
</styles>
<definitions>
<context id="done-line" style-ref="done" end-at-line-end="true" extend-parent="false">
<start>DONE</start>
</context>
<context id="todo">
<include>
<context ref="done-line"/>
</include>
</context>
</definitions>
</language>
Step 2) added a custom style to the file /usr/share/gtksourceview-2.0/styles/classic.xml by adding the line:
<style name="def:done" foreground="#88aa88" italic="true"/>
