I previously did a post on changing the Gnome Shell theme using the ThemeSelector Extension. While that method works great, themes can look weird if you are using other Gnome Shell extensions. This can be remedied by manually installing a Gnome Shell theme.
Install Gnome Shell Theme
Before installing a new Gnome Shell theme, you need to backup the default Gnome Shell theme. With the manual install method, you can only have one Gnome Shell theme at a time.
Open nautilus by pressing Alt + F2 and running:
Navigate to /usr/share/gnome-shell/ and copy the “theme” folder to a safe place. (This is necessary if you want to be able to revert back to the default Gnome Shell theme!)
Download a Gnome Shell theme you want to use. Rename its “gnome-shell” folder to “theme”. Copy the newly renamed theme folder to /usr/share/gnome-shell/, merging and replacing with the theme folder already located there.
Press ALT + F2 and run “r” to restart GNOME Shell, and to apply the new theme.