include: make header guards more sane The C++ standard[1] says: Each identifier that contains a double underscore __ or begins with an underscore followed by an uppercase letter is reserved to the implementation for any use. [1]: https://timsong-cpp.github.io/cppwp/n4659/lex.name#3.1

#ifndef MINORI_SYS_GLIB_DARK_THEME_H_
#define MINORI_SYS_GLIB_DARK_THEME_H_

namespace glib {

bool IsInDarkTheme();

}

#endif // MINORI_SYS_GLIB_DARK_THEME_H_