Reduce specificity of some CSS selectors for the popin

Some selectors for the popin are unnecessary long. Particularly selectors involving `.popin-header` ; `.popin-body` ; and `.popin-footer`. If we want to override these selectors, we easily get 4-level selectors.

This PR reduces the specificity of some selectors.
Instead of `.popin-container .popin .popin-body`, it is now `.popin-container .popin-body`