Hi there,
I need help with the update process of Zenphoto version 1.4.5.1 [b17846cb99] (Official Version) to the current version. What do I need to do exactly? If I have understood correctly I need to download and unpack only the current file? My Theme cutom copy of default works then no longer.

Actually you should be able to directly upgrade from 1.4.5.x to 1.4.6 as described here:
http://www.zenphoto.org/news/installation-and-upgrading

The "default" theme has been renamed to "basic" but is still the same theme.

However since this is a major version there are some small changes that will require updates to third party custom themes and plugins. Those are noted on the release post:
http://www.zenphoto.org/news/zenphoto-1.4.6

The error log should tell you what to change and you can workaround by using the deprecated functions plugin and config. However it is of course recommended to update your theme.