[FR]: Change theme based on system dark/light

[pm4rcin]

Brief description of the feature request

Hi, I use darkman utility to switch dark and light theme. This program exposes org.freedesktop.portal.Settings interface which allows to query for preffered color scheme. It can also read HighContrast preference from the system. Here's the documentation of the portal that many applications use and it works awesome. It would be great if it was implemented because otherwise theme is only set at program startup. https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Settings.html