Just one additional suggestion ... check plugins for compatibility with destination core version of Moodle.
Above can be scripted and checked via moosh.
addons.txt file list addons by shortname:
atto_morefontcolors
auth_saml2
Using moosh to check those using a bash shell script:
#!/bin/bash
#
echo 'Add-on listing: ';
cat ./addons.txt;
echo '---------------------';
for i in `cat ./addons.txt`
do
echo "Addon in que: $i";
moosh -n plugin-list |grep $i
done
Themes are plugins and probably should be the first thing to research about newest version of moodle core code.
Be a good 'boy scout' and be prepared for a theme snafu.
Add a line to config.php commented out but ready to use in the event theme not compatible with latest/greatest ...
// $CFG->theme='';
'SoS', Ken