Warning macro not readable with the default color theme (yellow letters on light-gray background)

see http://manager.xwiki.org/xwiki/bin/view/Sandbox/#HWarningMacro
or https://www.computer42.org/xwiki/bin/view/Sandbox/#HWarningMacro

The lack of contrast between yellow and light-gray make the warning macro unusable for content that should be readable.
In doubts this lead to using the error macro in cases where normally a warning is apposite.