Cannot use flamingo theme variables in skin extensions

Steps to reproduce:

• Create a stylesheet extension Main.MyCSS to use the color theme variables (set Parse content to Yes)

  #template('colorThemeInit.vm')
  p.test { 
  color: $theme.pageBackgroundColor;
  background-color: $theme.brand-info;
  }
  

• Create a test page to use the extension on demand:

  {{velocity}}
  
  $xwiki.ssx.use('Main.MyCSS')
  
  {{/velocity}}
  
  (% class="test" %)
  Test text that's in the class test, but does not use the flamingo color theme value
  

  Result: the brand-info property is not interpreted, while the old property works fine:

  p.test{color:#f5f5f5;background-color:$theme.brand-info}