UPPERCASE theme variables do not work.

[thet]

A manifest.cfg which contains UPPERCASE theme variables like the following:

manifest.cfg

[theme]
title = Test theme
...

[theme:parameters]
PLONE52 = python:True

which is used like that:

<?xml version="1.0" encoding="UTF-8"?>
<rules xmlns="http://namespaces.plone.org/diazo"
       xmlns:css="http://namespaces.plone.org/diazo/css"
       xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
[...]
    <rules if="$PLONE52">
[...]
    </rules>
</rules>

fails with:

    runtime error, element 'choose' [10360:0]
    Variable 'PLONE52' has not been declared. [0:0]
    Undefined variable [0:0]

When using a lowecase variable name, all is fine.