As you may have already noticed, the default site theme has changed. This was done both because of the comments based on the BlueMarine theme and in anticipation of an upcoming site upgrade and changes involving the way themes will be handled.
I and others like the user pictures apprearing in posts and comments as they do in BlueMarine. The new theme has those, a cleaner layout, and loads faster than previous themes.
The other themes will go away soon, but will remain available via your account preferences for a little while longer.
If you experience any issues with the new theme or miss something present in other themes, please let me know and I'll do my best to address those issues.