ohh I did not know that BuildRequires in the spec files downloads things for you .
Now it looks nicer
auto pTheme = Silica::Theme::instance();
g_nFontSizePx = pTheme->fontSizeTiny();
I made the following in the .pro file
INCLUDEPATH += $$[QT_HOST_PREFIX]/include/libsailfishsilica
LIBS += -lsailfishsilica
Or should that be done in some other way ??