I just searched the github repo but didn’t find anything in the code, so this is probably not changeable by theming. You’d probably have to take the NC theming, when you then add the code sources you have to manually change the logfile path (https://github.com/owncloud/client/search?utf8=✓&q=owncloudsync.log&type=) and build you own client.
Nextcloud announced client-side encryption with their own client, then it will hopefully easier to implement changes without referring to ownCloud each time: