2.5 to 4.2 is a big jump. So, without digging into exactly what you did, the first question is does it work with the core Boost theme?
Add the line...
$CFG->theme = 'boost';
...to your config.php file.
It may also be a good idea to https://docs.moodle.org/402/en/Developer_tools#Purge_all_caches
Add the line...
$CFG->theme = 'boost';
...to your config.php file.
It may also be a good idea to https://docs.moodle.org/402/en/Developer_tools#Purge_all_caches
the default theme is working, except in admin, the navigation items are not clickable
The smooth path is to uninstall the theme in the System administration > Plugins interface and then reinstall the theme. There are various ways of getting a theme. Ultimately they need to appear in the moodle/theme/ directly. Then Moodle detects them when the admin logs in next time and walks through the installation procedure (on the GUI).
The rough path is to just remove and replace the corresponding directory under moodle/theme/.
the theme issue was not resolved and i got a new one, whenever i go to site administration, it keeps asking for upgrade, the screenshot is attached
