No…  once you create your custom theme, you should never have to do it again…

And 99% of the time, you will not have to apply any theme changes we might make to our themes… we did provide them in case users want to, but we go to painstaking trouble to make sure any changes we make to simple press core will not break any current in use custom themes…

note, I did not say 100% because sometimes its just unavoidable…  in this case, it was not one of our changes…  WP updated to a major new version of jquery UI and tweaked the WP jquery ui css…  and the result was it could break some of our stuff…  so the theme updates become needed…

also, sometimes we release new functionality…  if you want to use the new functionality (ie new template tags) then you would have to edit your theme to take advantage of those template functions..

but if you are just as happy as can be with your forum and the css, you will rarely have to update your theme if we do our job..