Done ! The latest version of Dotclear is installed.
Smooth upgrade, no issue. As usually, I've simply applied the patch. No, I don't use the integrated update feature which imply some really unsecured configuration.
My theme works perfectly.
P.S. As for each new release, I also take care to manually copy all the images from the official archive.
p>Reminder:
$ tar czf save/blog-20160305.tgz blog
$ mysqldump -uroot -pxxx blog | gzip >save/remi-20160305.sql.gz
$ cd download
$ wget http://download.dotclear.org/patches/2.8.2-2.9.diff.gz
$ wget http://download.dotclear.org/latest/dotclear-2.9.tar.gz
$ cd ../blog
$ gzip -dc ../download/2.8.2-2.9.diff.gz | patch -p1
$ cd ..
$ tar xf download/dotclear-2.9.tar.gz
$ for i in $(find dotclear/ -name \*png); do cp $i ${i/dotclear/blog}; done