Fcitx5
Fcitx 5 is a generic input method framework.
Packages status
All packages: Fcitx5 package family
Install Fcitx5 on bullseye
For simplified Chinese users
Install fcitx5 and fcitx5-chinese-addons. Then logout and login again in your desktop environment. The input methods are not added to fcitx immediately. You need to configure it manually. You can run fcitx5-configtool from terminal or system tray.
apt install --install-recommends fcitx5 fcitx5-chinese-addons
Be sure to install recommended packages, it works for most people. But if you want a minimal set of packages to install, try (for KDE user):
apt install --no-install-recommends fcitx5 fcitx5-chinese-addons fcitx5-frontend-gtk3 fcitx5-frontend-qt5 fcitx5-module-kimpanel fcitx5-module-xorg kde-config-fcitx5
For traditional Chinese users
You can install fcitx5-chinese-addons too, it provides table method. But you can also install fcitx5-chewing for zhuyin.
GNOME
Gnome users should install gnome-shell-extension-kimpanel.
Fcitx5 works on Wayland, so you don't need to switch to Xorg.
Themes
Except the builtin theme, currently there's extra one available. fcitx5-material-color. See the screenshots at upstream.