You can revert everything they have changed. Depends on what you want to change, however.
You can redesign toolbars to however you want them to be from View > Toolbars. You can revert to the message list (the main area with messages) by displaying a message list header with View > Layouts > Message list header and opening the Message list display options at the top right of the message list header and selecting the list view.
Similarly, there are toggles for everything else. Just keep clicking until you get the desired look. Try searching on the internet when you do not know where to find some option.