Qt detect system dark/light mode?

Gertio

How dows Qt detect system dark/light mode?
I've tried

QGuiApplication::styleHints()->colorScheme();

but it returns

Qt::ColorScheme::Unknown

if called before the QGuiApplication constructor, so before I actually have an application.
Is there a chance I can get the ColorScheme of the OS itself before the app creation?

Christian Ehrlicher

@Gertio said in Qt detect system dark/light mode?:

Is there a chance I can get the ColorScheme of the OS itself before the app creation?

As you can see - no.

DerReisende

@Gertio If you want to figure out whether your windows system is dark mode or not you can use the following code:

static bool isDarkMode() {
        bool result{false};
        HKEY hKey;

        const auto lRes = RegOpenKeyExW(HKEY_CURRENT_USER,
                                        L"SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
                                        0, KEY_READ, &hKey);
        if (lRes == ERROR_SUCCESS) {
            DWORD usesLightTheme;
            const auto res = GetDWORDRegKey(hKey, L"AppsUseLightTheme", usesLightTheme);
            if (res == ERROR_SUCCESS) {
                //qDebug() << "Apps use light theme: " << usesLightTheme;
                result = !usesLightTheme;
            }
        } else
            qCritical() << "Failed to query dark mode";

        RegCloseKey(hKey);
        return result;
    }

LONG GetDWORDRegKey(HKEY hKey, const std::wstring &strValueName, DWORD &nValue) {
    nValue = -1;
    DWORD dwBufferSize(sizeof(DWORD));
    DWORD nResult(0);
    LONG nError = ::RegQueryValueEx(hKey,
                                    strValueName.c_str(),
                                    nullptr,
                                    nullptr,
                                    reinterpret_cast<LPBYTE>(&nResult),
                                    &dwBufferSize);
    if (ERROR_SUCCESS == nError) {
        nValue = nResult;
    }
    return nError;
}

JonB

@Gertio OOI, why do you want this setting prior to creating a QGuiApplication?

Gertio

@JonB well there are reasons, I want to set the default style based on wether the user has dark mode on or not, but I need to set it with

QGuiApplication::setStyle(QStyle *style)

before it is created to not break/disable CLI arguments (e.g. -style fusion) and environment variables.