It is possible to copy missing themes into Moodle after the upgrade, but this should happen before users touch the system, otherwise theme-related settings may be lost. Note: Only installed additional themes are updated automatically during the upgrade, NOT standard themes.
Because standard themes have been removed from Moodle 2.7, they have to be re-added.
If upgrading from earlier versions, you must upgrade to 2.2 as a first step.
Custom themes and themes from the Plugins Directory are not affected, unless they use one of the removed themes as a parent theme.
If this helps, the message feature has not been working as well.
It keeps erasing my contacts and when people send messages they do not show up in the message block or pop up.
See this tutorial if you are upgrading Moodle on cpanel.
It is a bit rough around the edges and is a little dated, but you should get the idea.
They immediately go into the message history without being read.