GNOME 42 has arrived! 
Gtk 4 theming and Libadwaita
Unfortunately, there will be no custom theming available for Libadwaita applications. However, once the Libadwaita Recoloring API is implemented, we should be able to add Maia to the color palette.
I suggest installing the adw-gtk-theme from our community repo to make all Gtk applications have a matching theme. In Tweaks, set the Legacy Applications theme to Adw or Adw-dark. I recommend setting the Shell theme to Default or another shell theme you prefer that has GNOME 42 support.
To automatically switch the system theme and legacy application theme when toggling between the Light and Dark style in Settings, install the Legacy (GTK3) Theme Scheme Auto Switcher extension (gnome-shell-extension-legacy-theme-auto-switcher).
Qt application theming
For Qt application theming, we have switched back to QGnomePlatform for GNOME 42. qt5ct and qt6ct (Qt Configuration Utilities) may be marked as orphans. If you want to use QGnomePlatform with the Adwaita Qt style, you’ll need remove or comment the QT_QPA_PLATFORMTHEME variable in /etc/environment. You’ll want both qgnomeplatform and qgnomeplatform-qt6 for Qt 5 and Qt 6 applications.
New Adwaita folder icon style
If you want the new Adwaita folder icon style with Papirus, you can use papirus-folders to set it:
For example, with Papirus-Dark:
papirus-folders -C adwaita --theme Papirus-Dark
Manjaro COSMIC status
For those using the Manjaro COSMIC packages, you will need to remove them. I’ve dropped them from the repos. Once System76 finishes their new Gtk 4 applications for Pop!_OS, I’ll see about adding them.
KNOWN ISSUES
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Bluetooth is not enabled by default due to “…changes between GNOME 41 and 42, fixing long-standing bugs which would cause adapters to be powered up when the user never asked for that.”
See: FS#74400 - [bluez] default config should set “AutoEnable=true” by default and After update to gnome-42 bluetooth is always “powered off” after reboot or suspend.
If you’re not using GNOME
If you want to change the theme of Libadwaita applications on another Gtk desktop like Xfce or Cinnamon, you can use the following commands:
Light Adwaita theme:
gsettings set org.gnome.desktop.interface color-scheme default
Dark Adwaita theme:
gsettings set org.gnome.desktop.interface color-scheme prefer-dark