Yes – the theme and plugins have been developed under an open-source licence so they are available for anyone to use or modify without charge or restriction.  The theme is a ‘Block Theme’ and so may not support all the features of your website if it currently uses a ‘Classic Theme’. 

If you already have plugins installed it is possible that the u3a plugins may conflict with their operation.  You should always test before installing them on a live website and ensure you have adequate backups to restore the site should any problems arise.