Hi.
I would like to know what kind of theme modifications that var stores.
Thx
It doesn't hold any modifications to the theme.
It is a unique number, that is changed whenever the theme is changed. It is used in the URL for all the theme-related stuff (e.g. CSS) so they can be served with caching headers to say "keep this for a long time, it won't change". If the contents of that stuff does change then you need to change the URL so that the browsers download the new CSS.
This number also changes when you do a 'Purge all caches'. When 'theme designer mode' is on it is set to '-1' such that there is no caching.