settings: Add color-scheme key

[alice-mkh]

Specify a key for getting the system's preferred color scheme.

Fixes #629

[TingPing]

Looks fine to merge once implementations exist.

[alice-mkh]

https://gitlab.gnome.org/GNOME/xdg-desktop-portal-gnome/-/merge_requests/12

Do we need an equivalent xdg-desktop-portal-gtk change? On one hand it's fairly GNOME-specific, but -gtk already has plenty of GNOME things in it. Since xdg-desktop-portal-gnome exists now, not sure what's the policy regarding that.

[alice-mkh]

elementary implementation: elementary/settings-daemon#38

[TingPing]

I'll leave the -gtk question to @GeorgesStavracas

[GeorgesStavracas]

Do we need an equivalent xdg-desktop-portal-gtk change?

I would say, yes.

[alice-mkh]

I would say, yes.

Ok - flatpak/xdg-desktop-portal-gtk#354

[lilianmoraru]

@Exalm, @GeorgesStavracas, @TingPing
Could you please also add support for High Contrast?
Microsoft does it very nicely: https://support.microsoft.com/en-us/windows/use-high-contrast-mode-in-windows-10-fedc744c-90ac-69df-aed5-c8a90125e696 , allowing users that maybe have problems seeing a particular color to switch to a different one.
Here they describe the event that apps will receive and how to read the user's desired colors, in order to avoid hardcoding them: https://docs.microsoft.com/en-us/windows/win32/w8cookbook/high-contrast-mode#solution

[alice-mkh]

We have it in GNOME, but I consider it a separate issue when it comes to having a standardized one - for example, elementary doesn't have it.

[lilianmoraru]

GNOME has a separate accessibility toggle with no dedicated D-Bus connection to listen on, for apps to handle it(and missing documentation).
It needs a more official/portable way of doing it(IMO). I thought that once you are here, this could be handled as well.

[alice-mkh]

I mean until recently (still is in 41) it was a theme - not even a flag. One thing at a time :p

[WhyNotHugo]

I'm trying to read this setting with non-flatpak'd apps (in my case, d-feet), but I get:

('g-dbus-error-quark: GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: '
 'Portal operation not allowed: Unable to open /proc/147042/root (9)')

Are native applications supposed to use a different mechanism to retrieve this value from the portal?

[swick]

IMO no, if it's not working for some reason that's a bug.

[alice-mkh]

[rusty-snake]

Fedora Linux 35; Workstation/GNOME; xdg-desktop-portal-gnome/xdg-desktop-portal-gtk; d-feet via dnf;

Works:

dbus-send --print-reply --dest=org.freedesktop.portal.Desktop /org/freedesktop/portal/desktop org.freedesktop.portal.Settings.Read string:'org.freedesktop.appearance' string:'color-scheme'

/usr/bin/d-feet works too => it's firejail.

[WhyNotHugo]

@rusty-snake Reported in #737