mimimi

Guys, you can track yourselves the changes if instead of pressing the Support tab you select the Developers tab and then click on Development log.

Here is the link to the overview of releases
http://themes.trac.ww.wp.xz.cn/log/twentythirteen/

Here is a link that compares the old v1.0 functions.php file with the
new v1.1
http://themes.trac.ww.wp.xz.cn/changeset?old_path=%2Ftwentythirteen%2F1.0%2Ffunctions.php&old=26830&new_path=%2Ftwentythirteen%2F1.1%2Ffunctions.php&new=28873

And this one is for screen.css
http://themes.trac.ww.wp.xz.cn/changeset?old_path=%2Ftwentythirteen%2F1.0%2Fstyle.css&old=26830&new_path=%2Ftwentythirteen%2F1.1%2Fstyle.css&new=28873

As you can see, the changes in functions.php are mostly cosmetic, a few comments here and there and some bad spelling / spaces. There’s nothing major.

The same is true for screen.css which has only 5 total changes mostly related to the attachment post sidebar.

However, if you just hacked your design directly in the twenty thirteen theme files, without making a child-theme, you will be forever locked in time as you will lose everything when updating, regardless how minor the changes.

Learn how to build a child theme. It takes like 5 minutes to setup.
There is official documentation for this:
http://codex.ww.wp.xz.cn/Child_Themes