You need to enable the User Themes GNOME Shell Extension.
Follow the link and flick the switch to "on" to enable it.
If you want to install it manually:
- Download and uncompress the zip file.
- Either move
flat-darkto your.themesdirectory in your home directory; or create a symbolic link to the folder (ctrl-shift drag does this in Nautilus). If the directory.themesdoes not exist create it. - Select "Flat Dark" as shell theme in gnomes Tweak Tool.
If you prefer to use the terminal:
mkdir ~/available-themes
mkdir ~/.themes
cd ~/available-themes
git clone https://github.com/nerdbeere/flat-dark-gnome-theme
ln -s ~/available-themes/flat-dark-gnome-theme/flat-dark ~/.themes/flat-dark
The icons I used together with an installation manual can be found here: https://github.com/daniruiz/Flat-Remix
Big shout out goes to https://github.com/daniruiz who build https://github.com/daniruiz/Flat-Remix-GNOME-theme from which I got the inspiration for this theme.
MIT License
Copyright (c) 2017 Julian Hollmann
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.