**bold** does not show bold, neither does *italiic* displays italic.
It does bold and italic by default with the current Geany 1.34.
header1=0xFFCB4F;0x1E1E1E;true;false this change will make # bold with specified background/front color, but not the header content, i.e. "Header1" in # Header1 stays non-bold and un-colored.
What is actually given a particular highlighting style is determined by the lexers in the Scintilla editing component Geany uses. Scintilla is a separate project so feature requests (or better still patches) should be addressed [there](https://www.scintilla.org/).