This is a Base16 template for FlatColor, a gtk3 theme by jasperro and deviantfero,
First download FlatColor theme from jasperro's repo to your .themes folder:
git clone https://github.com/jasperro/FlatColor ~/.themes/FlatColor
Before anything, you need to make a few changes in FlatColor so the base16 builder of your choice can inject to it.
Open ~/.themes/FlatColor/gtk-2.0/gtkrc with your favorite text editor.
Delete gtk-color-scheme and everything inside quotes following it, and replace with:
include "../colors2"
Now open .~/.themes/FlatColor/gtk-3.0/gtk.css.
Delete all in lines in section /* Default color scheme */, and replace with:
@import url("../colors3");
Do the same with ~/.themes/FlatColor/gtk-3.20/gtk.css.
Now using your injector (if you don't have one, check out flavours), and configure it so the template will be built to both ~/.themes/FlatColor/colors3 and ~/.themes/FlatColors/colors2, using the subtemplates gtk-2 and gtk-3, respectively.
Tip: Most gtk apps only update when restarted, but you can use gsettings or xsettingsd to change to some theme, and back to FlatColor so all applications re-theme without restart (Thanks jasperro for the workaround). (If you use flavours, you can set that as an hook).
First of all, create a dummy theme as a symlink to FlatColor (so your theme doesn't flash when changing):
ln -Ts ~/.themes/FlatColor ~/.themes/dummy
Now here's the two ways to reload your theme:
After installing it, edit your ~/.xsettings so it has the theme set:
Net/ThemeName "dummy"
Now just running xsettings should reload the theme! You can background the process, or make a systemd service to reload it more easily (check out my dotfiles if you want an example).
Use this command to reload:
gsettings set org.gnome.desktop.interface gtk-theme dummy && gsettings set org.gnome.desktop.interface gtk-theme FlatColor
If nothing happens, make sure /usr/lib/gsd-xsettings is running. You do need some gnome packages installed, though.