Advice for changing-theme-function for GNU/Linux system

[Jonas Damm]

Hi,

I wish to create my own function in emacs to switch my whole system from
dark to light mode.

I want to use the modus themes included in emacs, so i can toggle with
modus-themes-toggle.

However, I also want to change my systems GTK theme for other
applications like Evince. So i have to change the file
~/.config/gtk-3.0/settings.ini and replace "Adwaita" with "Adwaita-dark"
or the other way round. How would this be done? Do I need to call a
shell command with "sed", or can emacs to this natively by opening that
file, finding and replacing the String?

Thanks for any Ideas,
Jonas