Theming GTK4 Libadwaita applications

[choc1024]

Introduction

As it turns out, GTK 4 theming is totally possible on libadwaita applications! For example, most if not all themes by vinceliuice not only support this but also provide a script to do so. All you need is to modify some things in .config/gtk-4.0

Instructions

1. Get a theme that supports gtk 4. You can check they do by checking if they have a gtk-4.0 folder inside
2. Install that theme. ~/.themes is the most common place and recommended
3. Find the gtk.css and gtk-dark.css files of your theme: ususally located at ~/.themes/YOUR_THEME_NAME/gtk-4.0/gtk.css and ~/.themes/YOUR_THEME_NAME/gtk-4.0/gtk-dark.css
4. Create a symlink:
   Say your theme is at ~/.themes/ChromeOS.
   Execute the following commands:

cd ~/.config/gtk-4.0/
mv gtk.css gtk.css.bak # Creates a backup. It's fine if it says the file doesn't exist
mv gtk-dark.css gtk-dark.css.bak # Fine if it says the file doesn't exist
ln -s ~/.themes/ChromeOS/gtk-4.0/gtk.css gtk.css
ln -s ~/.themes/ChromeOS/gtk-4.0/gtk-dark.css gtk-dark.css

5. Done! You will probably need to reopen most apps tho

Notes and Conclusion

I have tested this on Ubuntu and it works for all my GTK 4 applications, including those using libadwaita, like Nautilus, GNOME Tweaks, GNOME Software and GNOME Settings. However, it only works for those that are not sandboxed. If the application comes from flatpak, this will not work. I'm not sure about snap though.

I'd also like to ask the Linux Mint developers to somewhat implement this as a native feature instead of the user having to do this kind of things.

EDIT

Turns out, it is also possible to make flatpak applications use the theme. In order to do so however, we need to expose the ~/.themes folder to the flatpak applications. I have tried using other folders but ~/.themes seems to be the only one that actually works at the moment. However, this is a serious security risk. So what I have done instead is to make the ~/.themes read-only for non-sudo applications:

sudo chown -R root:root ~/.themes
sudo chmod -R a-w ~/.themes
flatpak override --user --filesystem=~/.themes:ro
flatpak override --user --env=GTK_THEME={INSERT YOUR THEME HERE}

Please note this is just a temporary workaround. For now, it seems to have applied my theme to all flatpak applications I have (except krita of course because it doesn't use GTK at all). Definitely not ideal since it requires modifying permissions and sudo, but it's OK for me.

[Murat-Karakaya]

Nice work! I would perhaps try it one day

[gaetan-1960]

...and .. what happens here with all the .css renaming etc once updates rolled out? .. Changes gone after the update?? .. That's why I never start manipulations like this, despite if they work or not...

[Murat-Karakaya]

Valid argument. But I often find myself doing stuff just for the sake of it. Plus, it isn't much of a hastle really.

[choc1024]

I don't think updates can affect the theme, and if they decide to change css names, that's gonna be in GTK 5.

[choc1024]

nwg-look seems to automatically apply this if a theme supports it, but it has to be manually compiled on non-Arch-based distros