How to recognize that an application is running in dark theme on Linux?

Question

I've developed an application which uses qscintilla as a text editor. I also implemented custom lexer to highlight language specific keywords. So far styles for highlighted keywords are hardcoded in mine application and it looks quite ok in the default theme on Windows/Linux(Ubuntu)/Mac.

The problems appears when user chooses a dark theme (on Linux). Depending on QScintilla version some parts of editor do reflect current theme colors while other don't. Moreover mine custom styles render dark blue letters on dark grey background.

I'm looking for some Qt class, which will allow me access of the current system theme. I do not want to define styles for mine application widgets. I want to know what is system default non-proportional font, what is it's size, color, ... If I knew that dark scheme is used I would choose complementary colors for keyword highlighting.

I checked docs for QStyle, QPlatformTheme and other qt classes and it seems to me that these serve more for defining of new styles, then for describing the current style.

Answer 1

For the system colours, you can use the group/role of the QPalette class.

For the system fonts, you can create a QFont using e.g. "Serif", "Sans Serif", "Monospace", etc with an appropriate style hint to discover the defaults.

NB:

From the Qt Docs:

Warning: Some styles do not use the palette for all drawing, for instance, if they make use of native theme engines. This is the case for both the Windows Vista and the macOS styles.

Answer 2

Here is some python code using QPalette that works for me on Linux:

label = QLabel("am I in the dark?")
text_hsv_value = label.palette().color(QPalette.WindowText).value()
bg_hsv_value = label.palette().color(QPalette.Background).value()
dark_theme_found = text_hsv_value > bg_hsv_value