CSS Themes will be now rendered as external CSS files rather than internal. What this means for you is that if you make use of one of the pre-set themes, such as the Dark Redux theme, and changes are made to it, your CSS Theme will automatically reflect the changes.
However, due to the old CSS system, users' CSS Themes were saved as internal CSS, so if you'd like to switch over to the current CSS pre-set Themes and have your CSS pre-set Theme automatically reflect the changes, just go to CSS Theme Editor, and select the pre-set you'd like to use. The CSS Theme will reflect all changes from that point on.
If you decide to stay with a pre-set, I'd like to remind you: if you make changes to the CSS, it effectively becomes a custom theme, which means if the pre-set you were using is updated, changes will not show for you since you're using your own custom theme.
I'm working on setting it up so members can "subscribe" to other members' custom CSS Themes. Expect to see this implemented by tomorrow night!
If you do not want to use a pre-set, but want to switch your CSS over to the new system so when I implement the member-css subscription code, other members can subscribe to your custom CSS Theme, just go into CSS Theme Editor, and click "Save Custom". This will result in your switching over to the new system.