Simple theme with nothing extra to use on a regular basis. Theme tested on Manjaro and Fedora.
The theme is still developing and it would be nice if you reported all the bugs in issues.

Gtk4 apps

Gtk3 apps

Flatpak apps

Gnome 43 apps

Gnome-shell

Light

Dark

Download theme here.

Move theme folders to ~/.themes

To install the Gtk4 theme move the contents of gtk4.0 to ~/.config/gtk4.0

To install themes on Flatpak apps use these commands:

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

sudo flatpak override --env=GTK_THEME=MonoTheme

or

sudo flatpak override --env=GTK_THEME=MonoThemeDark

IMPORTANT❗️ Take a snapshot of the system before use!
If you want the gnome-shell theme to extend to the lock and login screen, move the gnome-shell-theme.gresource file from gnome-shell folder to /usr/share/gnome-shell/ with a replacement and restart system with ALT+F2 and enter r for Xorg or reboot/log out for Wayland session.
Command:

sudo cp gnome-shell-theme.gresource /usr/share/gnome-shell

I recommend saving the gresource file from the folder to a safe place beforehand, if will need to be returned. Go to /usr/share/gnome-shell and open terminal.
Example:

sudo cp gnome-shell-theme.gresource ~/Documents

Install Firefox Theme

Firefox

This is a mix of Adwaita symbolic icons and new icons that we made together with my friend. Icons match and don't get out from the style of Adwaita icons.
Install Icon Theme

Icons

To change the light or dark theme of Gtk3 apps and Gnome-shell use Gnome Tweaks or Night Theme Switcher (choose the MonoTheme for day and MonoThemeDark for night variant in the Themes tab and change the theme color by switching style in Settings > Appearance).
The dark and light appearance of Gtk4 is changed by renaming the desired file to gtk.css in .config/gtk4.0 or you can try the script I made for automatic theme switcher in gtk4 and gtk3/4 flatpak apps. Go to Night Theme Switcher and select Commands tab and paste this script.
Sunrise:

flatpak override --env=GTK_THEME=MonoTheme --user & cd ~/.config/gtk-4.0 ; mv gtk.css gtk2.css ; mv gtk-dark.css gtk.css ; mv gtk2.css gtk-dark.css

Sunset:

flatpak override --env=GTK_THEME=MonoThemeDark --user & cd ~/.config/gtk-4.0 ; mv gtk.css gtk2.css ; mv gtk-dark.css gtk.css ; mv gtk2.css gtk-dark.css

Thanks to this script, you can change the theme of apps by switching dark and light theme in the settings, but gtk4 and gtk4 flatpak apps will be updated only when the app window is reopened. If ALL the steps are completed correctly, then the theme color will change for ALL applications in the system, write your questions in issues.

To remove a theme, follow all the steps above in reverse order