Start by thinking about the separation between content and design.
Typically, a member's area is confidential with respect to content, but not design. So you'll want to wall off only the content. Stylesheets, javascript and any images that control the look and feel of the site are design elements (typically). Some images might, of course, be member-only content. Anything that handles design should be shared for two reasons.
1. Maintenance. It's a hassle to keep two versions of the same thing.
2. Performance. You want to hit the browser cache as often as possible for stylesheets and javascript and images.
So I would probably have something like this if the styling is very simple:
base.css
members.css
But they would be in the same folder and both publicly accessible.
If the site is going to be complex, I would look into something like SMACSS or Object Oriented CSS, breaking things into lots of files and using SASS or LESS to generate a final, compressed CSS file or files.
SMACCS - from Jonathon Snook who has done some of the biggest redesigns around (like Yahoo! mail). [
smacss.com...]
OOCSS from Stubbornella (Nicole Sullivan - [
github.com...]
Drupal has traditionally had abysmal, bloated markup that is hard to maintain. The Drupal community put together a sort of task force to tackle the issue of how to implement CSS, drawing on the work of Snook, Sullivan and others and have put together an interesting CSS standard for complex projects
[
drupal.org...]
I find it a little easier to see put together like that than reading from Snook and Sullivan et al.
