I was on Reddit when I saw a very cool icons to install. Its theme is named Flat-Remix.
My first attempt was to follow the instructions explained on its Reddit page. Terminal prompt and the repository installation. Didn't work. So, I downloaded the zip file and uncompressed the Flat-Remix folder in ~/.themes and didn't work either. Therefore, I restarted my computer hoping to get the good news, and nothing. There's none Flat-Remix theme or icon on System Settings > Themes.
I thought, after reading a little, that it could be something related with the obsolescence of the theme in my OS. So I checked out the version of Linux Mint of the guy who shared the theme and, same as mine, Linux Mint 19.1 Cinnamon. After that, I asked to the guy how did he do the installation, and he answered that he just made the package install via the terminal prompt, just as I did.
Recently, I had an issue with the locales, but It's look like pretty solved and I don't think that it's affecting something.