We just rolled out the new MantisHub release featuring our brand new modern UI. The new UI is based on Bootstrap and provides MantisHub with a fresh modern look.
Administators will have the ability to switch between modern and classic UI via the Manage page. The switch will apply to apply to all users for that MantisHub.
At the moment, the modern UI is only available via MantisHub, with the plan to integrate into Mantis Bug Tracker version 2.0.