fix: properly handle light/dark theme switch

[caybro]

What does the PR do

• under Qt6, we can now listen to the styleHints color scheme property changes and react accordingly
• remove the hack of detecting the current dark theme by checking the hsl lightness of the window; also removes propagating the systemPalette object thru the hierarchy
• as a result, we can get rid of the dedicated C++ OSThemeEvent

Fixes #17764
Fixes #14856

Needs status-im/dotherside#95
Needs status-im/nimqml#67

Affected areas

App/Settings

Architecture compliance

• I am familiar with the application architecture and agreed good practices.
  My PR is consistent with this document: QML Architecture Guidelines

Screencapture of the functionality

Zaznam.obrazovky.z.2025-07-08.20-04-35.mp4

Storybook:

Impact on end user

Settings/Appearance/System should follow system light/dark theme changes

[status-im-auto]

Jenkins Builds

❔	Commit	#️⃣	Finished (UTC)	Duration	Platform	Result
✔️	814575d	#1	2025-07-08 18:04:13	~7 min	tests/nim	📄log
✔️	814575d	#1	2025-07-08 18:07:50	~10 min	macos/aarch64	🍎dmg
✔️	814575d	#1	2025-07-08 18:09:46	~12 min	tests/ui	📄log
✔️	814575d	#1	2025-07-08 18:12:59	~15 min	linux/x86_64	📦tgz
✔️	814575d	#1	2025-07-08 18:29:53	~32 min	windows/x86_64	💿exe
✔️	e6b2d68	#2	2025-07-09 15:19:28	~7 min	tests/nim	📄log
✔️	e6b2d68	#2	2025-07-09 15:19:36	~7 min	macos/aarch64	🍎dmg
✔️	e6b2d68	#2	2025-07-09 15:24:58	~12 min	tests/ui	📄log
✔️	e6b2d68	#2	2025-07-09 15:25:09	~12 min	linux/x86_64	📦tgz
✔️	e6b2d68	#2	2025-07-09 15:46:49	~34 min	windows/x86_64	💿exe

[micieslak]

Cool! I tested on Linux and Mac and it works really nice.

The thing that is questionable to me is using system theme on login page even if dark/light is explicitly specified in settings:

Screencast.from.09.07.2025.11.49.50.webm

I think that we should respect user's choice also on the login page.

The other thing is why we need to keep this on the nim side. I think that property should not be synced - it's specific to given device, therefore regular qt's Settings should be suitable here.

[caybro]

Cool! I tested on Linux and Mac and it works really nice.

The thing that is questionable to me is using system theme on login page even if dark/light is explicitly specified in settings:

Screencast.from.09.07.2025.11.49.50.webm
I think that we should respect user's choice also on the login page.

The other thing is why we need to keep this on the nim side. I think that property should not be synced - it's specific to given device, therefore regular qt's Settings should be suitable here.

It's done that since before the login, there are no user preferences available (no user/profile logged in yet -> no settings), so we opt to always follow the system theme. Nothing we could do/fix here really

[caybro]

Cool! I tested on Linux and Mac and it works really nice.
The thing that is questionable to me is using system theme on login page even if dark/light is explicitly specified in settings:
Screencast.from.09.07.2025.11.49.50.webm
I think that we should respect user's choice also on the login page.
The other thing is why we need to keep this on the nim side. I think that property should not be synced - it's specific to given device, therefore regular qt's Settings should be suitable here.

It's done that since before the login, there are no user preferences available (no user/profile logged in yet -> no settings), so we opt to always follow the system theme. Nothing we could do/fix here really

    Component.onCompleted: {
        console.info(">>> %1 %2 started, using Qt version %3".arg(Qt.application.name).arg(Qt.application.version).arg(SystemUtils.qtRuntimeVersion()))

        Theme.changeTheme(Theme.Style.System);

^^ this is main.qml, during Onboarding but the settings are stored in localAppSettings which is not around at this point

[jrainville]

Awesome!

It's funny how the color switch when using the System theme is super smooth and when switching manually in the app is very sudden, but it's not a real issue

[caybro]

Awesome!

It's funny how the color switch when using the System theme is super smooth and when switching manually in the app is very sudden, but it's not a real issue

Yeah because it's the system/OS compositor doing the color transition, not us 😆