A GTK theme based on the colours of Sainnhe's great theme: Everforest for Neovim
the VinceLiuice's Awesome GTK Themes and the creativity of Gusbemacbe's: Suru Plus Icon Theme.

Before installing the themes, make sure to install the following necessary packages: sassc murrine-engine and gnome-themes-extra packages for the correct rendering of themes.

Here are some commands to install on some distributions.

• On Fedora run:

 sudo dnf install gtk-murrine-engine

• On OpenSUSE run:

 sudo zypper install gtk2-engine-murrine

• On Arch run:

sudo pacman -S gtk-engine-murrine

• On Debian and derivatives run:

sudo apt install gtk2-engines-murrine

• Download the themes packs and extract them
• Move the extracted files to the following paths:
  • For GTK3: ~/.themes In this path you must move the entire theme folder.
  • For GTK4: ~/.config/gtk-4.0 The files to move to this path can be found inside the theme directory in the gtk-4.0 folder, copy only the assets, gtk.css and gtk-dark.css files or create a symlinks.

• For GTK3, apply themes from Gnome Tweaks, Refine or Tuner
• For GTK4 applications it is only necessary to have moved the assets, gtk.css and gtk-dark.css files to the ~/.config/gtk-4.0 path, and if you notice that the theme has not been applied, just close and reopen the application.

• Override flatpak themes to ~/.themes:

sudo flatpak override --filesystem=$HOME/.themes

• Override flatpak icons to ~/.icons:

sudo flatpak override --filesystem=$HOME/.icons

• Override flatpak themes to ~/.config/gtk-4.0 locally:

flatpak override --user --filesystem=xdg-config/gtk-4.0

• Override flatpak themes to ~/.config/gtk-4.0 globally:

sudo flatpak override --filesystem=xdg-config/gtk-4.0

Run the following command in the terminal for a general installation

./install.sh

The ./install.sh allows some specific options like:

./install.sh --tweaks medium macos outline float -t green -l

To customise the name of the theme, you can use the -n parameter, e.g.

./install.sh -n "Everforest-Float-Border" --tweaks outline float

-d, --dest DIR          Specify destination directory (Default: ~/.themes)

-n, --name NAME         Specify theme name (Default: Everforest)

-t, --theme VARIANT...  Specify theme accent color variant(s) [default|green|grey|orange|pink|purple|red|teal|yellow|all] (Default: blue)

-c, --color VARIANT...  Specify color variant(s) [light|dark] (Default: All variants)

-s, --size VARIANT...   Specify size variant [standard|compact] (Default: standard variant)

-l, --libadwaita        Link installed gtk-4.0 theme to config folder for all libadwaita app use this theme

-r, --remove,

-u, --uninstall         Uninstall/Remove installed themes or links

--tweaks                Specify versions for tweaks
                        1. [medium|soft]  Medium|Soft ColorSchemes version
                        2. black          Blackness color version
                        3. float          Floating gnome-shell panel style
                        4. outline        Windows with 2px outline style
                        5. macos          macOS style windows button

-h, --help              Show help

This is just to clarify doubts about the abbreviations of the Themes

Thanks to @telometto for the alternative to the application of themes in Flatpak.
Thanks to @f1yn for the solution to the active and inactive borders in the new version of Cinnamon.
Thanks to @eeeXun for the hint to solve the bug in Mate Desktop window control buttons.
Thanks to @Icy-Thought,@D3vil0p3r and to those who have packaged these themes for NIX and AUR.