You can make use of these options either by invoking zellij with zellij options [OPTION] or binding them in the configuration file.

Eg. zellij options --simplified-ui true is equivalent to simplified_ui: true in the config file.

default-modedefault_modenormalThe first mode on startup.
default-shelldefault_shell$SHELLThe default shell.
default-layoutdefault_layoutdefaultThe name of the default layout.
mouse-modemouse_modetrueEnable mouse mode.
disable-mouse-mode-falseDisable mouse mode.
simplified-uisimplified_uifalseRequest the Plugins to use a more compatible ui.
pane-framespane_framestrueDisplay frames around the panes.
no-pane-frames-falseDisable display of frames around the panes.
on-force-closeon_force_closedetachWhat to do when receiving a SIGTERM, SIGINT, SIGQUIT or SIGHUP.
themethemedefaultSwitch to a theme configured under the themes section.
scroll-buffer-sizescroll_buffer_size10000The number of lines zellij stores for each pane in the scroll back buffer.
_envemptyA map of key value pairs for environment variables.
copy-commandcopy_command_Command to execute when copying text, instead of using OSC52 (for example wl-copy, xclip, pbcopy).
copy-clipboardcopy_clipboardsystemDestination selection buffer (x11/linux) for OSC52 copy, can be system or primary.
copy-on-selectcopy_on_selecttrueAutomatically copy when selecting text. If false, the Copy action should be bound in normal and scroll mode.
scrollback-editorscrollback_editor-Path to the editor to use to edit scrollback (eg. /usr/bin/vim)

A small example:

default_mode: locked
mouse_mode: false