Hello David,
Not entirely sure if I understand you here but are you referring to the custom css section shown here,
http://take.ms/gIXAU ? If you have placed your css codes here, they are actually stored in,
JOOMLA_ROOT/templates/JOOMLA_TEMPLATE/html/com_easyblog/styles/custom.css
It doesn't matter which theme of EasyBlog that you use as this file will be automatically be loaded on the page.