Theming Qt and Gtk

Heavily based on ArchLinux Wiki.

Unify Qt5/Qt6 and GTK2/GTK3 design.

Use recommended 96 DPI. Scaling is hell, don't try it!

Of course, this depends on your personal preference. If you pick

GNOME the default Adwaita (dark) theme is very pleasing and
for all other scenarios, Arc (Darker) looks great.

Theme

Qt applications

To unify look in GTK and Qt applications, install

qt5ct
qt6ct

Important: In any environment (your shell env and probably envvars of environment.d) source, e.g. .zshenv, set export QT_QPA_PLATFORMTHEME="qt5ct" (which is also forward compatible with qt6ct).

If your choice is Adwaita

For Adwaita install the following

adwaita-qt5
adwaita-qt6

Start each and do the same: qt5ct and qt6ct
- select and dialogs to Default
- select Default as Palette
- select Adwaita (Dark) as theme

If your choice is another theme, e.g. Arc (Dark/Darker)

Install the following packages depending on your choice

arc-gtk-theme
arc-kde
kvantum
# Arc or any other kvantum supported theme
kvantum-theme-arc

Start each and do the same: qt5ct and qt6ct
- select and dialogs to Default
- select Default as Palette
Use kvantummanager and select theme KvArc... (or what you like to set)

Gtk 3 applications

Set your proper theme via respective utilities like nwg-look and/or GNOME Tweaks for GTK 4.

Gtk 2

Set your proper theme via respective utilities like nwg-look.

Install the Gtk 2 variant of your theme first, e.g. adwaita-dark or arc-theme.

Font

Noto Sans Regular (11) seems great. Set it everywhere.

Set them via qt5ct, qt6ct and via respective utilities like nwg-look and/or GNOME Tweaks for GTK.

In addition, adapting fontconfig might be useful. See KB/Linux/Desktop/Fonts.

Icons

papirus-icon-theme looks great, but you should also install some fallbacks.

papirus-icon-theme
arc-icon-theme
adwaita-icon-theme