I had ubuntu 16.04 earlier with gnome environment and apple look alike theme with the help the help of tweak tool . Now I have upgraded to 18.04 . But I am experiencing problems with the apple theme, like some buttons not appear , icon names at the top bar not aligned correctly , etc. How to get default theme for 18.04
SnapOverflow
It may be easiest for you at this point to reset your theme and icons to the Ubuntu factory defaults with two command in the terminal:
Open a terminal, copy and paste these two commands and your theme and icons will be reset to those of a freshly installed Ubuntu system. To change to other installed themes later on, you can use Tweak Tool.