[themes] some questions about themes

Hello

Is there a theme that is configurable and customizable? Let me tell you the background of my question: I want to let users customize their view like gmail does.

My 2nd question is about changing themes. Whenever I change the theme, I lose all the dimensions of the frames and menus. I don't know how themes were developed in drupal, but changing from 1 theme to another should not ideally hurt the look of the site.

And my 3rd question is about sub-theme. I keep seeing this term especially in Zen theme. Does it mean I can run a Garland theme and inside it I can define a Zen theme for a view? OR does it mean I can create a new theme out of the parent theme that is always Zen theme?

thank you for clarifying these 3 points.

best regards,