Here are 2 nice themes for Gnome shell.
GNOME Shell – Gaia
1- Backup your theme directory first.
2- Extract the zip file and rename “gnome-shell” directory to “theme”
3- Copy to /usr/share/gnome-shell/. This path is usually where GNOME Shell is installed.
4- Restart GNOME Shell by Alt, F2, then type r and press enter.