3
Yes, I agree with Burning.
Thought all module works with all themes, the dislplay location (or / blocks) may change ( example the menu could be a drop-down from top in one theme where as it could be static on the other theme). So you may have to do little customisation.
Customising theme can be very time consuming. So first concentrate with the module/functionality that you need on your web page and make them work with default theme.
Deepak