New port: x11-themes/flat-remix-icon-themes: Flat Remix is an icon theme inspired by material design

Flat Remix is an icon theme inspired by material design. It is mostly flat
using a colorful palette with some shadows, highlights, and gradients for some
depth.

https://github.com/daniruiz/flat-remix

PR: 246608
Submitted by: Alexander Vereeken <Alexander88207@protonmail.com>
Differential Revision: https://reviews.freebsd.org/D24921