Apply dark / light mode without a restart

[mesonium]

Is your feature request related to a problem? Please describe.
Applying a dark or light theme currently requires to go to the settings, to (un)check a checkbox and to restart Xournal++.
Doing this twice a day, in the mornings and in the evenings, can be cumbersome.

Describe the solution you'd like
It'd be great if Xournal++ followed the system theme automatically.

E.g. when the global theme on Plasma is changed from Breeze Twilight to Breeze Dark, other GTK apps (Firefox/Thunderbird/Gajim/etc.) automatically switch from Adwaita to Adwaita Dark and vice versa, without the need of a restart.