Other modules can override $vars['styles'] after unlimited_css_preprocess_page() has run.

Other modules can override $vars['styles'] after unlimited_css_preprocess_page() has run. If this happens, unlimited_css does not work.

Solution:
- Put a copy of the rebuilt $vars['styles'] into $vars['styles_unlimited_css']. We can assume that the latter will not be killed by other modules.
- Custom themes can now use this value instead of $vars['styles'].