ThemeChanger

This app is a theme changing utility for Linux, BSDs, and whatnots. It lets the user change GTK 2/3/4, Kvantum, icon and cursor themes, even for libadwaita apps, edit GTK CSS with live preview, and set some related options. It also lets the user install icon and widget theme archives.

Features

Set the GTK 3 theme, sync the GTK2, GTK4, Kvantum themes with it or choose another one for each of these toolkits
Set the icon theme
Set the cursor theme, and tweak the cursor's size
Set all these themes with a special searchable selector with previews for GTK3, icon and cursor themes
Set various options like whether buttons have images or not
Instantaneously apply your setting changes to the running applications in GNOME, Cinnamon, Mate, XFCE, LXDE using lxsession, or using xsettingsd (you must download xsettingsd and run it in the background) for those that don't use GTK desktop environments
Edit GTK CSS with instantaneous feedback of the changes made
Install new widget or icon themes from archives available e.g. at https://gnome-look.org

Installation

Arch Linux & friends

Install the themechanger-git package from the AUR the way you like it. For instance, I like it this way:

yay -S themechanger-git

Nix(OS)

Install the themechanger package from nixpkgs, like this:

nix-env -iA nixos.themechanger # On NixOS
nix-env -iA nixpkgs.themechanger # On any other platform

From source

Make sure you have installed PyGobject, Gtk3, GLib (for the app running); headers thereof, glib-compile-resources, meson, ninja (for the installation process). For reference, this is how you get the required dependencies on Ubuntu:

sudo apt install python3-gi libglib2.0 libgtk-3.0 # runtime dependencies
sudo apt install meson gcc libglib2.0-dev-bin python3-dev python-gi-dev libxcursor-dev libgdk-pixbuf2.0-dev # build-time dependencies

In the folder with the source (obtainable e.g. by running git clone https://github.com/ALEX11BR/ThemeChanger) run meson build, then ninja -C build install, and you're ready to go!

TODOs

Add more options to set (see further reference)
Add theme remover
Add a client for OCS-compatible websites like gnome-look.org, tailored for downloading (and automatically installing) themes (see API reference and a reference project)
Copy global theme files locally so that flatpak apps can use them (maybe?)
Option for configuring named colors
Use a cleaner CSS live preview method (probably it's not possible)
Clean the code overall