We (BETS Moodle Support Team) have been receiving some complaints about the new default Moodle user interface. Actually, the only reason this theme (RemUI) was selected as the default theme is that, it is responsive, i.e. it works well on mobile devices as well.
It is possible for each user (either an instructor or a student) revert to the classical Moodle user interface (theme).
A tutorial for changing the user interface theme is available here. All one needs to do is follow the steps given on this page and choose the “Clean” theme. This theme is very similar to old Moodle user interface, plus more options related to the new functionalities available with the current Moodle version.
Please note that teachers can enforce students to use a certain theme for each of their courses.