[NEW PORT] x11-themes/sierra-gtk-themes: MacOS High Sierra like theme for GTK 3, GTK 2 and Gnome-Shell etc

[Alexander88207_protonmail.com]

Summary

Sierra is a Mac OSX like theme for GTK 3, GTK 2 and Gnome-Shell which supports GTK 3 and GTK 2 based desktop environments like Gnome, Pantheon, XFCE, Mate, etc.

WWW: https://github.com/vinceliuice/Sierra-gtk-theme

Related to PR247023

Test Plan

Poudriere 12.1-Release amd64: Ok
Portlint: looks fine

Tested with Mate & XFCE

[Alexander88207_protonmail.com]

Using shebangfix instead of @bash

[lwhsu]

New port added, thanks!

Some minor points:

• Please submit patch against the root of working copy (ports/)
• Use portlint -abct to check